Scanner 类中的 next()和 nextLine()方法有什么区别？

next() can read the input only till the space. It can't read two words separated by a space. Also, next() places the cursor in the same line after reading the input.

nextLine() reads input including space between the words (that is, it reads till the end of line \n). Once the input is read, nextLine() positions the cursor in the next line.

For reading the entire line you can use nextLine().

From javadocs

next() Returns the next token if it matches the pattern constructed from the specified string. nextLine() Advances this scanner past the current line and returns the input that was skipped.

Which one you choose depends which suits your needs best. If it were me reading a whole file I would go for nextLine until I had all the file.

From the documentation for Scanner:

A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace.

From the documentation for next():

A complete token is preceded and followed by input that matches the delimiter pattern.

From JavaDoc:

• A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace.
• next(): Finds and returns the next complete token from this scanner.
• nextLine(): Advances this scanner past the current line and returns the input that was skipped.

So in case of "small example<eol>text" next() should return "small" and nextLine() should return "small example"

I always prefer to read input using nextLine() and then parse the string.

Using next() will only return what comes before the delimiter (defaults to whitespace). nextLine() automatically moves the scanner down after returning the current line.

A useful tool for parsing data from nextLine() would be str.split("\\s+").

String data = scanner.nextLine();
String[] pieces = data.split("\\s+");
// Parse the pieces

For more information regarding the Scanner class or String class refer to the following links.

Scanner: http://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html

String: http://docs.oracle.com/javase/7/docs/api/java/lang/String.html

next() and nextLine() methods are associated with Scanner and is used for getting String inputs. Their differences are...

next() can read the input only till the space. It can't read two words separated by space. Also, next() places the cursor in the same line after reading the input.

nextLine() reads input including space between the words (that is, it reads till the end of line \n). Once the input is read, nextLine() positions the cursor in the next line.

import java.util.Scanner;


public class temp
{
public static void main(String arg[])
{
Scanner sc=new Scanner(System.in);
System.out.println("enter string for c");
String c=sc.next();
System.out.println("c is "+c);
System.out.println("enter string for d");
String d=sc.next();
System.out.println("d is "+d);
}
}

Output:

enter string for c abc def
c is abc

enter string for d

d is def

If you use nextLine() instead of next() then

Output:

enter string for c

ABC DEF
c is ABC DEF
enter string for d

GHI
d is GHI

What I have noticed apart from next() scans only upto space where as nextLine() scans the entire line is that next waits till it gets a complete token where as nextLine() does not wait for complete token, when ever '\n' is obtained(i.e when you press enter key) the scanner cursor moves to the next line and returns the previous line skipped. It does not check for the whether you have given complete input or not, even it will take an empty string where as next() does not take empty string

public class ScannerTest {


public static void main(String[] args) {


Scanner sc = new Scanner(System.in);
int cases = sc.nextInt();
String []str = new String[cases];
for(int i=0;i<cases;i++){
str[i]=sc.next();
}
}


}

Try this program by changing the next() and nextLine() in for loop, go on pressing '\n' that is enter key without any input, you can find that using nextLine() method it terminates after pressing given number of cases where as next() doesnot terminate untill you provide and input to it for the given number of cases.

In short: if you are inputting a string array of length t, then Scanner#nextLine() expects t lines, each entry in the string array is differentiated from the other by enter key.And Scanner#next() will keep taking inputs till you press enter but stores string(word) inside the array, which is separated by whitespace.

Lets have a look at following snippet of code

    Scanner in = new Scanner(System.in);
int t = in.nextInt();
String[] s = new String[t];


for (int i = 0; i < t; i++) {
s[i] = in.next();
}

when I run above snippet of code in my IDE (lets say for string length 2),it does not matter whether I enter my string as

Input as :- abcd abcd or

Input as :-

abcd

abcd

Output will be like abcd

abcd

But if in same code we replace next() method by nextLine()

    Scanner in = new Scanner(System.in);
int t = in.nextInt();
String[] s = new String[t];


for (int i = 0; i < t; i++) {
s[i] = in.nextLine();
}

Then if you enter input on prompt as - abcd abcd

Output is :-

abcd abcd

and if you enter the input on prompt as abcd (and if you press enter to enter next abcd in another line, the input prompt will just exit and you will get the output)

Output is:-

abcd

• Just for another example of Scanner.next() and nextLine() is that like below : nextLine() does not let user type while next() makes Scanner wait and read the input.

   Scanner sc = new Scanner(System.in);
  
  
  do {
  System.out.println("The values on dice are :");
  for(int i = 0; i < n; i++) {
  System.out.println(ran.nextInt(6) + 1);
  }
  System.out.println("Continue : yes or no");
  } while(sc.next().equals("yes"));
  // while(sc.nextLine().equals("yes"));
  

The key point is to find where the method will stop and where the cursor will be after calling the methods.

All methods will read information which does not include whitespace between the cursor position and the next default delimiters(whitespace, tab, \n--created by pressing Enter). The cursor stops before the delimiters except for nextLine(), which reads information (including whitespace created by delimiters) between the cursor position and \n, and the cursor stops behind \n.

For example, consider the following illustration:

|23_24_25_26_27\n

| -> the current cursor position

_ -> whitespace

stream -> Bold (the information got by the calling method)

See what happens when you call these methods:

nextInt()

read 23|_24_25_26_27\n

nextDouble()

read 23_24|_25_26_27\n

next()

read 23_24_25|_26_27\n

nextLine()

read 23_24_25_26_27\n|

After this, the method should be called depending on your requirement.

A scanner breaks its input into tokens using a delimiter pattern, which is by default known the Whitespaces.

Next() uses to read a single word and when it gets a white space,it stops reading and the cursor back to its original position. NextLine() while this one reads a whole word even when it meets a whitespace.the cursor stops when it finished reading and cursor backs to the end of the line. so u don't need to use a delimeter when you want to read a full word as a sentence.you just need to use NextLine().

 public static void main(String[] args) {
// TODO code application logic here
String str;
Scanner input = new Scanner( System.in );
str=input.nextLine();
System.out.println(str);
}

I also got a problem concerning a delimiter. the question was all about inputs of

1. enter your name.
2. enter your age.
3. enter your email.
4. enter your address.

The problem

1. I finished successfully with name, age, and email.
2. When I came up with the address of two words having a whitespace (Harnet street) I just got the first one "harnet".

The solution

I used the delimiter for my scanner and went out successful.

Example

 public static void main (String args[]){
//Initialize the Scanner this way so that it delimits input using a new line character.
Scanner s = new Scanner(System.in).useDelimiter("\n");
System.out.println("Enter Your Name: ");
String name = s.next();
System.out.println("Enter Your Age: ");
int age = s.nextInt();
System.out.println("Enter Your E-mail: ");
String email = s.next();
System.out.println("Enter Your Address: ");
String address = s.next();


System.out.println("Name: "+name);
System.out.println("Age: "+age);
System.out.println("E-mail: "+email);
System.out.println("Address: "+address);
}

The basic difference is next() is used for gettting the input till the delimiter is encountered(By default it is whitespace,but you can also change it) and return the token which you have entered.The cursor then remains on the Same line.Whereas in nextLine() it scans the input till we hit enter button and return the whole thing and places the cursor in the next line. **

        Scanner sc=new Scanner(System.in);
String s[]=new String[2];
for(int i=0;i<2;i++){
s[i]=sc.next();
}
for(int j=0;j<2;j++)
{
System.out.println("The string at position "+j+ " is "+s[j]);
}

**

Try running this code by giving Input as "Hello World".The scanner reads the input till 'o' and then a delimiter occurs.so s[0] will be "Hello" and cursor will be pointing to the next position after delimiter(that is 'W' in our case),and when s[1] is read it scans the "World" and return it to s[1] as the next complete token(by definition of Scanner).If we use nextLine() instead,it will read the "Hello World" fully and also more till we hit the enter button and store it in s[0]. We may give another string also by using nextLine(). I recommend you to try using this example and more and ask for any clarification.

Both functions are used to move to the next Scanner token.

The difference lies in how the scanner token is generated

next() generates scanner tokens using delimiter as White Space

nextLine() generates scanner tokens using delimiter as '\n' (i.e Enter key presses)

The difference can be very clear with the code below and its output.

    public static void main(String[] args) {
List<String> arrayList = new ArrayList<>();
List<String> arrayList2 = new ArrayList<>();
Scanner input = new Scanner(System.in);


String product = input.next();


while(!product.equalsIgnoreCase("q")) {
arrayList.add(product);
product = input.next();
}


System.out.println("You wrote the following products \n");
for (String naam : arrayList) {
System.out.println(naam);
}


product = input.nextLine();
System.out.println("Enter a product");
while (!product.equalsIgnoreCase("q")) {
arrayList2.add(product);
System.out.println("Enter a product");
product = input.nextLine();
}


System.out.println();
System.out.println();
System.out.println();
System.out.println();


System.out.println("You wrote the following products \n");
for (String naam : arrayList2) {
System.out.println(naam);
}
}

Output:

Enter a product
aaa aaa
Enter a product
Enter a product
bb
Enter a product
ccc cccc ccccc
Enter a product
Enter a product
Enter a product
q
You wrote the following products


aaa
aaa
bb
ccc
cccc
ccccc
Enter a product
Enter a product
aaaa aaaa aaaa
Enter a product
bb
Enter a product
q








You wrote the following products




aaaa aaaa aaaa
bb

Quite clear that the default delimiter space is adding the products separated by space to the list when next is used, so each time space separated strings are entered on a line, they are different strings. With nextLine, space has no significance and the whole line is one string.

The next() methods [There are several next() methods such as nextInt(),nextDouble() etc.] scan for tokens (you can think of this as a word), and the nextLine() method reads from the Scanner's current location until the beginning of the next line.

I saw that other authors had highlighted it as well, so I thought I'd explain it using an simple example to demonstrate how to utilize them to prevent problems with reading inputs through stdin (Standard Inputs Stream).

One of the most important thing you should have understood in order to avoid confusions is each line of multi-line input comprises an invisible separator that indicates the end of a line of input which is known as a delimiter.

When you call Scanner functions that read tokens (for example, next(), nextInt(),nextDouble() and so on), the Scanner reads and returns the next token. When you read a whole line (nextLine(),readLine()), it reads from the current position to the start of the next line. As a result, if there are no characters between the end of the most recent read and the beginning of the next line, a call to nextLine() may return an empty string.

As an example, consider the following as input:

a b c
d e
f
g

The following steps illustrate how a certain sequence of calls to a Scanner object, will read the provided input:

1. A call to scan.next(); returns the next token, a.
2. A call to scan.next(); returns the next token, b.
3. A call to scan.nextLine(); returns the next token, c. It's important to note that the scanner returns a space and a letter, because it's reading from the end of the last token until the beginning of the next line.
4. A call to scan.nextLine(); returns the contents of the next line, d e.
5. A call to scan.next(); returns the next token, f.
6. A call to scan.nextLine(); returns everything after f until the beginning of the next line; because there are no characters there, it returns an empty String.
7. A call to scan.nextLine(); returns g.

I hope this may help you to prevent a lot of misunderstandings and issues cause when coding.