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java - Does String.intern() change reference of Original String JDK7

问题描述:

Possible Duplicate:

intern() behaving differently in Java 6 and Java 7

While doing example for this question

I noticed a strange behaviour of intern() method when I call intern() method on String thereafter I can use == operator for the Original String.

JavaDoc of intern() method:

Returns a canonical representation for the string object.

A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

Above Javadoc does not say that the orginal string gets changed. So why this program prints okay when test is the input.

import java.util.Scanner;

public class Test {

public static void main(String[] args) {

Scanner user_input = new Scanner(System.in);

String username;

System.out.print("username: ");

username = user_input.next();

// Even if I do not assign returned string for comparison still it compares

// okay else it does not compare

username.intern();

if (username == "test") {

System.out.println("okay");

}

else {

System.out.println("not okay");

}

}

}

网友答案:

String.intern() returns the intern()ed String. This is because a method cannot change a reference passed to it.

So why this program prints okay when test is the input.

It prints okay because the intern()ed string is the first time this String is seen so it becomes the one String literal for "test". It is not the String which is changed, but the object which is used for "test" which is changed.

Try instead.

String te = "te", st = "st";
// "test".length();
String username = te + st;
username.intern();
System.out.println("String object the same is: "+ (username == "test"));

In this case, the output in Java 7 update 7 is

String object the same is: test

but run this on Java 6 update 32 or uncomment the line so "test" is seen first and you get.

String object the same is: false
网友答案:

str.intern() checks to see if there is already the same object in String pool and if so, reuses it.

网友答案:

Below line give you the input form user.

    username = user_input.next();

After that you are calling username.intern(); which returns the String literal of username
Now in your condition statement

if (username == "test") 

your variable username contain your String literal object from string pool and "test" is also a String literal so jvm is going to give you same reference of String test which is already created for username that means username and "test" are the same object. And in java == operated check reference for the object so in this case it will return true thats why its prints okay

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