如何在Java中使用比较器进行排序

我学会了如何使用比较器，但我在使用比较器时遇到了困难。我的代码中有错误：

Exception in thread "main" java.lang.ClassCastException: New.People cannot be cast to java.lang.Comparable
at java.util.Arrays.mergeSort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at java.util.Collections.sort(Unknown Source)
at New.TestPeople.main(TestPeople.java:18)

下面是我的代码：

import java.util.Comparator;


public class People implements Comparator {
private int id;
private String info;
private double price;


public People(int newid, String newinfo, double newprice) {
setid(newid);
setinfo(newinfo);
setprice(newprice);
}


public int getid() {
return id;
}


public void setid(int id) {
this.id = id;
}


public String getinfo() {
return info;
}


public void setinfo(String info) {
this.info = info;
}


public double getprice() {
return price;
}


public void setprice(double price) {
this.price = price;
}


public int compare(Object obj1, Object obj2) {
Integer p1 = ((People) obj1).getid();
Integer p2 = ((People) obj2).getid();


if (p1 > p2) {
return 1;
} else if (p1 < p2){
return -1;
} else {
return 0;
}
}
}

import java.util.ArrayList;
import java.util.Collections;


public class TestPeople {
public static void main(String[] args) {
ArrayList peps = new ArrayList();


peps.add(new People(123, "M", 14.25));
peps.add(new People(234, "M", 6.21));
peps.add(new People(362, "F", 9.23));
peps.add(new People(111, "M", 65.99));
peps.add(new People(535, "F", 9.23));


Collections.sort(peps);


for (int i = 0; i < peps.size(); i++){
System.out.println(peps.get(i));
}
}
}

我相信这与比较方法中的铸造有关，但我一直在尝试，但仍然找不到解决方案。

659838 次浏览
小开

使用People implements Comparable<People>代替；这定义了People的自然排序。

此外，还可以定义Comparator<People>，但People implements Comparator<People>不是正确的做法。

Collections.sort的两个过载是不同的：

  • <T extends Comparable<? super T>> void sort(List<T> list)
    • 使用对象的自然顺序对Comparable对象进行排序
  • <T> void sort(List<T> list, Comparator<? super T> c)
    • 使用兼容的Comparator对任何内容进行排序

如果尝试对Comparator进行排序，则会将两者混淆（这也是为什么Person implements Comparator<Person>没有意义的原因）。同样，要使用Collections.sort，您需要满足以下条件之一：

  • 类型必须__abc0（使用1-arg__abc1）
  • 必须提供该类型的Comparator（使用2-argssort）

相关问题

还有，不要在新代码中使用原始类型。原始类型是不安全的，提供它只是为了兼容性。

也就是说，代替这个：

ArrayList peps = new ArrayList(); // BAD!!! No generic safety!

你应该像这样使用类型安全泛型声明：

List<People> peps = new ArrayList<People>(); // GOOD!!!

然后你会发现您的代码甚至无法编译！，这将是一件好事，因为代码中有一些错误（Person不implements Comparable<Person>），但是因为您使用了原始类型，所以编译器没有检查这一点。，相反，你在运行时得到一个ClassCastException！

这将说服您在新代码中始终使用类型安全的泛型类型。总是。

另请参阅

小开

您希望实现Comparable，而不是Comparator.您需要实现CompareTo方法。不过你很接近了。比较器是“第三方”比较例程。可比较的是这个物体可以与另一个物体相比较。

public int compareTo(Object obj1) {
People that = (People)obj1;
Integer p1 = this.getId();
Integer p2 = that.getid();


if (p1 > p2 ){
return 1;
}
else if (p1 < p2){
return -1;
}
else
return 0;
}

请注意，您可能需要在此处检查getId的空值。以防万一。

小开

您的示例类有几个棘手的问题：

  • 当它有一个price和info（更多的是物体，而不是人）时，它被称为人。
  • 当将一个类命名为某物的复数形式时，它意味着它是不止一个事物的抽象。

总之，下面是如何使用Comparator<T>的演示：

public class ComparatorDemo {


public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("Joe", 24),
new Person("Pete", 18),
new Person("Chris", 21)
);
Collections.sort(people, new LexicographicComparator());
System.out.println(people);
Collections.sort(people, new AgeComparator());
System.out.println(people);
}
}


class LexicographicComparator implements Comparator<Person> {
@Override
public int compare(Person a, Person b) {
return a.name.compareToIgnoreCase(b.name);
}
}


class AgeComparator implements Comparator<Person> {
@Override
public int compare(Person a, Person b) {
return a.age < b.age ? -1 : a.age == b.age ? 0 : 1;
}
}


class Person {


String name;
int age;


Person(String n, int a) {
name = n;
age = a;
}


@Override
public String toString() {
return String.format("{name=%s, age=%d}", name, age);
}
}

编辑

等效的Java8演示如下所示：

public class ComparatorDemo {


public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("Joe", 24),
new Person("Pete", 18),
new Person("Chris", 21)
);
Collections.sort(people, (a, b) -> a.name.compareToIgnoreCase(b.name));
System.out.println(people);
Collections.sort(people, (a, b) -> a.age < b.age ? -1 : a.age == b.age ? 0 : 1);
System.out.println(people);
}
}
小开

这里有一个超短的模板，可以立即进行排序：

Collections.sort(people, new Comparator<Person>() {
@Override
public int compare(final Person lhs, Person rhs) {
// TODO return 1 if rhs should be before lhs
//      return -1 if lhs should be before rhs
//      return 0 otherwise (meaning the order stays the same)
}
});

如果很难记住，试着记住它类似于（就数字的符号而言）：

 lhs-rhs

这是在你想按升序排序的情况下：从最小的数字到最大的数字。

小开

为了完整起见，下面是一个简单的一行程序__abc0方法：

Collections.sort(people, new Comparator<Person>() {
@Override
public int compare(Person lhs, Person rhs) {
return Integer.signum(lhs.getId() - rhs.getId());
}
});
小开

您应该使用重载的sort（peps，new people（））方法

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;


public class Test
{
public static void main(String[] args)
{
List<People> peps = new ArrayList<>();


peps.add(new People(123, "M", 14.25));
peps.add(new People(234, "M", 6.21));
peps.add(new People(362, "F", 9.23));
peps.add(new People(111, "M", 65.99));
peps.add(new People(535, "F", 9.23));


Collections.sort(peps, new People().new ComparatorId());


for (int i = 0; i < peps.size(); i++)
{
System.out.println(peps.get(i));
}
}
}


class People
{
private int id;
private String info;
private double price;


public People()
{


}


public People(int newid, String newinfo, double newprice) {
setid(newid);
setinfo(newinfo);
setprice(newprice);
}


public int getid() {
return id;
}


public void setid(int id) {
this.id = id;
}


public String getinfo() {
return info;
}


public void setinfo(String info) {
this.info = info;
}


public double getprice() {
return price;
}


public void setprice(double price) {
this.price = price;
}


class ComparatorId implements Comparator<People>
{


@Override
public int compare(People obj1, People obj2) {
Integer p1 = obj1.getid();
Integer p2 = obj2.getid();


if (p1 > p2) {
return 1;
} else if (p1 < p2){
return -1;
} else {
return 0;
}
}
}
}
小开 
public static Comparator<JobSet> JobEndTimeComparator = new Comparator<JobSet>() {
public int compare(JobSet j1, JobSet j2) {
int cost1 = j1.cost;
int cost2 = j2.cost;
return cost1-cost2;
}
};
小开
可以通过以下方式优化

解决方案： 首先，使用私有内部类作为字段的作用域，作为封闭类TestPeople，这样类的实现人员就不会暴露在外部世界中。这可以理解为创建一个API，它需要一个排序的人员列表。 其次，使用Lamba表达式（Java8）减少了代码，从而减少了开发工作量

因此，代码如下：

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;


public class TestPeople {
public static void main(String[] args) {
ArrayList<People> peps = new ArrayList<>();// Be specific, to avoid
// classCast Exception


TestPeople test = new TestPeople();


peps.add(test.new People(123, "M", 14.25));
peps.add(test.new People(234, "M", 6.21));
peps.add(test.new People(362, "F", 9.23));
peps.add(test.new People(111, "M", 65.99));
peps.add(test.new People(535, "F", 9.23));


/*
* Collections.sort(peps);
*
* for (int i = 0; i < peps.size(); i++){
* System.out.println(peps.get(i)); }
*/


// The above code can be replaced by followin:


peps.sort((People p1, People p2) -> p1.getid() - p2.getid());


peps.forEach((p) -> System.out.println(" " + p.toString()));


}


private class People {
private int id;


@Override
public String toString() {
return "People [id=" + id + ", info=" + info + ", price=" + price + "]";
}


private String info;
private double price;


public People(int newid, String newinfo, double newprice) {
setid(newid);
setinfo(newinfo);
setprice(newprice);
}


public int getid() {
return id;
}


public void setid(int id) {
this.id = id;
}


public String getinfo() {
return info;
}


public void setinfo(String info) {
this.info = info;
}


public double getprice() {
return price;
}


public void setprice(double price) {
this.price = price;
}
}
}
小开

Java8添加了一种制作比较器的新方法，它减少了您必须编写的代码量，比较器.正在比较。还可以查看比较器。反转

这里有一个样本

import org.junit.Test;


import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;


import static org.junit.Assert.assertTrue;


public class ComparatorTest {


@Test
public void test() {
List<Person> peopleList = new ArrayList<>();
peopleList.add(new Person("A", 1000));
peopleList.add(new Person("B", 1));
peopleList.add(new Person("C", 50));
peopleList.add(new Person("Z", 500));
//sort by name, ascending
peopleList.sort(Comparator.comparing(Person::getName));
assertTrue(peopleList.get(0).getName().equals("A"));
assertTrue(peopleList.get(peopleList.size() - 1).getName().equals("Z"));
//sort by name, descending
peopleList.sort(Comparator.comparing(Person::getName).reversed());
assertTrue(peopleList.get(0).getName().equals("Z"));
assertTrue(peopleList.get(peopleList.size() - 1).getName().equals("A"));
//sort by age, ascending
peopleList.sort(Comparator.comparing(Person::getAge));
assertTrue(peopleList.get(0).getAge() == 1);
assertTrue(peopleList.get(peopleList.size() - 1).getAge() == 1000);
//sort by age, descending
peopleList.sort(Comparator.comparing(Person::getAge).reversed());
assertTrue(peopleList.get(0).getAge() == 1000);
assertTrue(peopleList.get(peopleList.size() - 1).getAge() == 1);
}


class Person {


String name;
int age;


Person(String n, int a) {
name = n;
age = a;
}


public String getName() {
return name;
}


public int getAge() {
return age;
}


public void setName(String name) {
this.name = name;
}


public void setAge(int age) {
this.age = age;
}
}






}
小开

下面是一个比较器的示例，它适用于任何返回Comparable的零arg方法。JDK或库中是否存在这样的东西？

import java.lang.reflect.Method;
import java.util.Comparator;


public class NamedMethodComparator implements Comparator<Object> {


//
// instance variables
//


private String methodName;


private boolean isAsc;


//
// constructor
//


public NamedMethodComparator(String methodName, boolean isAsc) {
this.methodName = methodName;
this.isAsc = isAsc;
}


/**
* Method to compare two objects using the method named in the constructor.
*/
@Override
public int compare(Object obj1, Object obj2) {
Comparable comp1 = getValue(obj1, methodName);
Comparable comp2 = getValue(obj2, methodName);
if (isAsc) {
return comp1.compareTo(comp2);
} else {
return comp2.compareTo(comp1);
}
}


//
// implementation
//


private Comparable getValue(Object obj, String methodName) {
Method method = getMethod(obj, methodName);
Comparable comp = getValue(obj, method);
return comp;
}


private Method getMethod(Object obj, String methodName) {
try {
Class[] signature = {};
Method method = obj.getClass().getMethod(methodName, signature);
return method;
} catch (Exception exp) {
throw new RuntimeException(exp);
}
}


private Comparable getValue(Object obj, Method method) {
Object[] args = {};
try {
Object rtn = method.invoke(obj, args);
Comparable comp = (Comparable) rtn;
return comp;
} catch (Exception exp) {
throw new RuntimeException(exp);
}
}


}
小开

不要浪费时间自己实现排序算法。而应使用Collections.sort()对数据进行排序。

小开

两处更正：

  1. 您必须创建People对象的ArrayList：

    ArrayList<People> preps = new ArrayList<People>();

  2. After adding the objects to the preps, use: 

    Collections.sort(preps, new CompareId());

Also, add a CompareId class as: 

class CompareId implements Comparator {
public int compare(Object obj1, Object obj2) {
People t1 = (People)obj1;
People t2 = (People)obj2;


if (t1.marks > t2.marks)
return 1;
else
return -1;
}
}
小开

下面是我对一个简单的比较工具的回答

public class Comparator {
public boolean isComparatorRunning  = false;
public void compareTableColumns(List<String> tableNames) {
if(!isComparatorRunning) {
isComparatorRunning = true;
try {
for (String schTableName : tableNames) {
Map<String, String> schemaTableMap = ComparatorUtil.getSchemaTableMap(schTableName);
Map<String, ColumnInfo> primaryColMap = ComparatorUtil.getColumnMetadataMap(DbConnectionRepository.getConnectionOne(), schemaTableMap);
Map<String, ColumnInfo> secondaryColMap = ComparatorUtil.getColumnMetadataMap(DbConnectionRepository.getConnectionTwo(), schemaTableMap);
ComparatorUtil.publishColumnInfoOutput("Comparing table : "+ schemaTableMap.get(CompConstants.TABLE_NAME));
compareColumns(primaryColMap, secondaryColMap);
}
} catch (Exception e) {
ComparatorUtil.publishColumnInfoOutput("ERROR"+e.getMessage());
}
isComparatorRunning = false;
}
}


public void compareColumns(Map<String, ColumnInfo> primaryColMap, Map<String, ColumnInfo> secondaryColMap) {
try {
boolean isEqual = true;
for(Map.Entry<String, ColumnInfo> entry : primaryColMap.entrySet()) {
String columnName = entry.getKey();
ColumnInfo primaryColInfo = entry.getValue();
ColumnInfo secondaryColInfo = secondaryColMap.remove(columnName);
if(secondaryColInfo == null) {
// column is not present in Secondary Environment
ComparatorUtil.publishColumnInfoOutput("ALTER", primaryColInfo);
isEqual = false;
continue;
}
if(!primaryColInfo.equals(secondaryColInfo)) {
isEqual = false;
// Column not equal in secondary env
ComparatorUtil.publishColumnInfoOutput("MODIFY", primaryColInfo);
}
}
if(!secondaryColMap.isEmpty()) {
isEqual = false;
for(Map.Entry<String, ColumnInfo> entry : secondaryColMap.entrySet()) {
// column is not present in Primary Environment
ComparatorUtil.publishColumnInfoOutput("DROP", entry.getValue());
}
}


if(isEqual) {
ComparatorUtil.publishColumnInfoOutput("--Exact Match");
}
} catch (Exception e) {
ComparatorUtil.publishColumnInfoOutput("ERROR"+e.getMessage());
}
}


public void compareTableColumnsValues(String primaryTableName, String primaryColumnNames, String primaryCondition, String primaryKeyColumn,
String secTableName, String secColumnNames, String secCondition, String secKeyColumn) {
if(!isComparatorRunning) {
isComparatorRunning = true;
Connection conn1 = DbConnectionRepository.getConnectionOne();
Connection conn2 = DbConnectionRepository.getConnectionTwo();


String query1 = buildQuery(primaryTableName, primaryColumnNames, primaryCondition, primaryKeyColumn);
String query2 = buildQuery(secTableName, secColumnNames, secCondition, secKeyColumn);
try {
Map<String,Map<String, Object>> query1Data = executeAndRefactorData(conn1, query1, primaryKeyColumn);
Map<String,Map<String, Object>> query2Data = executeAndRefactorData(conn2, query2, secKeyColumn);


for(Map.Entry<String,Map<String, Object>> entry : query1Data.entrySet()) {
String key = entry.getKey();
Map<String, Object> value = entry.getValue();
Map<String, Object> secondaryValue = query2Data.remove(key);
if(secondaryValue == null) {
ComparatorUtil.publishColumnValuesInfoOutput("NO SUCH VALUE AVAILABLE IN SECONDARY DB "+ value.toString());
continue;
}
compareMap(value, secondaryValue, key);
}


if(!query2Data.isEmpty()) {
ComparatorUtil.publishColumnValuesInfoOutput("Extra Values in Secondary table "+ ((Map)query2Data.values()).values().toString());
}
} catch (Exception e) {
ComparatorUtil.publishColumnValuesInfoOutput("ERROR"+e.getMessage());
}
isComparatorRunning = false;
}
}


private void compareMap(Map<String, Object> primaryValues, Map<String, Object> secondaryValues, String columnIdentification) {
for(Map.Entry<String, Object> entry : primaryValues.entrySet()) {
String key = entry.getKey();
Object value = entry.getValue();
Object secValue = secondaryValues.get(key);
if(value!=null && secValue!=null && !String.valueOf(value).equalsIgnoreCase(String.valueOf(secValue))) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Secondary Table does not match value ("+ value +") for column ("+ key+")");
}
if(value==null && secValue!=null) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Values not available in primary table for column "+ key);
}
if(value!=null && secValue==null) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Values not available in Secondary table for column "+ key);
}
}
}


private String buildQuery(String tableName, String column, String condition, String keyCol) {
if(!"*".equalsIgnoreCase(column)) {
String[] keyColArr = keyCol.split(",");
for(String key: keyColArr) {
if(!column.contains(key.trim())) {
column+=","+key.trim();
}
}
}
StringBuilder queryBuilder = new StringBuilder();
queryBuilder.append("select "+column+" from "+ tableName);
if(!ComparatorUtil.isNullorEmpty(condition)) {
queryBuilder.append(" where 1=1 and "+condition);
}
return queryBuilder.toString();
}


private Map<String,Map<String, Object>> executeAndRefactorData(Connection connection, String query, String keyColumn) {
Map<String,Map<String, Object>> result = new HashMap<String, Map<String,Object>>();
try {
PreparedStatement preparedStatement = connection.prepareStatement(query);
ResultSet resultSet = preparedStatement.executeQuery();
resultSet.setFetchSize(1000);
if (resultSet != null && !resultSet.isClosed()) {
while (resultSet.next()) {
Map<String, Object> columnValueDetails = new HashMap<String, Object>();
int columnCount = resultSet.getMetaData().getColumnCount();
for (int i=1; i<=columnCount; i++) {
String columnName = String.valueOf(resultSet.getMetaData().getColumnName(i));
Object columnValue = resultSet.getObject(columnName);
columnValueDetails.put(columnName, columnValue);
}
String[] keys = keyColumn.split(",");
String newKey = "";
for(int j=0; j<keys.length; j++) {
newKey += String.valueOf(columnValueDetails.get(keys[j]));
}
result.put(newKey , columnValueDetails);
}
}


} catch (SQLException e) {
ComparatorUtil.publishColumnValuesInfoOutput("ERROR"+e.getMessage());
}
return result;
}

}

相同的实用工具

public class ComparatorUtil {


public static Map<String, String> getSchemaTableMap(String tableNameWithSchema) {
if(isNullorEmpty(tableNameWithSchema)) {
return null;
}
Map<String, String> result = new LinkedHashMap<>();
int index = tableNameWithSchema.indexOf(".");
String schemaName = tableNameWithSchema.substring(0, index);
String tableName = tableNameWithSchema.substring(index+1);
result.put(CompConstants.SCHEMA_NAME, schemaName);
result.put(CompConstants.TABLE_NAME, tableName);
return result;
}


public static Map<String, ColumnInfo> getColumnMetadataMap(Connection conn, Map<String, String> schemaTableMap) {
try {
String schemaName = schemaTableMap.get(CompConstants.SCHEMA_NAME);
String tableName = schemaTableMap.get(CompConstants.TABLE_NAME);
ResultSet resultSetConnOne = conn.getMetaData().getColumns(null, schemaName, tableName, null);
Map<String, ColumnInfo> resultSetTwoColInfo = getColumnInfo(schemaName, tableName, resultSetConnOne);
return resultSetTwoColInfo;
} catch (SQLException e) {
e.printStackTrace();
}
return null;
}


/* Number Type mapping
* 12-----VARCHAR
* 3-----DECIMAL
* 93-----TIMESTAMP
* 1111-----OTHER
*/
public static Map<String, ColumnInfo> getColumnInfo(String schemaName, String tableName, ResultSet columns) {
try {
Map<String, ColumnInfo> tableColumnInfo = new LinkedHashMap<String, ColumnInfo>();
while (columns.next()) {
ColumnInfo columnInfo = new ColumnInfo();
columnInfo.setSchemaName(schemaName);
columnInfo.setTableName(tableName);
columnInfo.setColumnName(columns.getString("COLUMN_NAME"));
columnInfo.setDatatype(columns.getString("DATA_TYPE"));
columnInfo.setColumnsize(columns.getString("COLUMN_SIZE"));
columnInfo.setDecimaldigits(columns.getString("DECIMAL_DIGITS"));
columnInfo.setIsNullable(columns.getString("IS_NULLABLE"));
tableColumnInfo.put(columnInfo.getColumnName(), columnInfo);
}
return tableColumnInfo;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}


public static boolean isNullOrEmpty(Object obj) {
if (obj == null)
return true;
if (String.valueOf(obj).equalsIgnoreCase("NULL"))
return true;
if (obj.toString().trim().length() == 0)
return true;
return false;
}






public static boolean isNullorEmpty(String str) {
if(str == null)
return true;
if(str.trim().length() == 0)
return true;
return false;
}


public static void publishColumnInfoOutput(String type, ColumnInfo columnInfo) {
String str = "ALTER TABLE "+columnInfo.getSchemaName()+"."+columnInfo.getTableName();
switch(type.toUpperCase()) {
case "ALTER":
if("NUMBER".equalsIgnoreCase(columnInfo.getDatatype()) || "DATE".equalsIgnoreCase(columnInfo.getDatatype())) {
str += " ADD ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype()+");";
} else {
str += " ADD ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype() +"("+columnInfo.getColumnsize()+"));";
}
break;
case "DROP":
str += " DROP ("+columnInfo.getColumnName()+");";
break;
case "MODIFY":
if("NUMBER".equalsIgnoreCase(columnInfo.getDatatype()) || "DATE".equalsIgnoreCase(columnInfo.getDatatype())) {
str += " MODIFY ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype()+");";
} else {
str += " MODIFY ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype() +"("+columnInfo.getColumnsize()+"));";
}
break;
}
publishColumnInfoOutput(str);
}


public static Map<Integer, String> allJdbcTypeName = null;


public static Map<Integer, String> getAllJdbcTypeNames() {
Map<Integer, String> result = new HashMap<Integer, String>();
if(allJdbcTypeName != null)
return allJdbcTypeName;
try {
for (Field field : java.sql.Types.class.getFields()) {
result.put((Integer) field.get(null), field.getName());
}
} catch (Exception e) {
e.printStackTrace();
}
return allJdbcTypeName=result;
}


public static String getStringPlaces(String[] attribs) {
String params = "";
for(int i=0; i<attribs.length; i++) { params += "?,"; }
return params.substring(0, params.length()-1);
}

}

列信息类

public class ColumnInfo {
private String schemaName;
private String tableName;
private String columnName;
private String datatype;
private String columnsize;
private String decimaldigits;
private String isNullable;
小开

为了完整起见。

使用Java8

people.sort(Comparator.comparingInt(People::getId));

如果你想在descending order

people.sort(Comparator.comparingInt(People::getId).reversed());
小开

如果您使用的是Java 8，那么最好使用下面这样的代码：

Comparator<People> comparator = Comparator.comparing(People::getName);

然后简单地使用：

Collections.sort(list, comparator);

如果您使用的是Java7或更低版本，那么您可以通过实现compare方法使用比较器来定制排序顺序。

例如：

import java.util.Comparator;


public class PeopleNameComparator implements Comparator<People> {
@Override
public int compare(People people1, People people2) {
return people1.getName().compareTo(people2.getName());
}
}

然后像这样简单地使用：

Collections.sort(list, new PeopleNameComparator);
小开

这是比较器的Lambda版本。这将根据长度对字符串列表进行排序。

Collections.sort(str, (str1, str2) -> {
if(str1.length() < str2.length())
return 1;
else if(str2.length() < str1.length())
return -1;
else
return 0;
});

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