返回String.intern()的说明 [英] The return of String.intern() explained

问题描述

考虑:

String s1 = new StringBuilder("Cattie").append(" & Doggie").toString();
System.out.println(s1.intern() == s1); // true why?
System.out.println(s1 == "Cattie & Doggie"); // true another why?

String s2 = new StringBuilder("ja").append("va").toString();
System.out.println(s2.intern() == s2); // false

String s3 = new String("Cattie & Doggie");
System.out.println(s3.intern() == s3); // false
System.out.println(s3 == "Cattie & Doggie"); // false

我感到困惑，为什么

I got confused why they are resulting differently by the returned value of String.intern() which says:

调用intern方法时，如果池中已经包含等于equals(Object)方法确定的此String对象的字符串，则返回池中的字符串.否则，将此String对象添加到池中，并返回对此String对象的引用.

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.

特别是在这两项测试之后:

Especially after these two tests:

assertFalse("new String() should create a new instance", new String("jav") == "jav");
assertFalse("new StringBuilder() should create a new instance",
    new StringBuilder("jav").toString() == "jav");

我曾经读过一篇文章，谈论一些在其他所有事物之前都被拘禁的特殊字符串，但是现在这真是一个模糊.

I once read a post talking about some special strings interned before everything else, but it's a real blur now.

如果有一些字符串是 pre-interned ，是否有一种方法可以获取它们的列表?我只是对它们的功能感到好奇.

If there are some strings pre-interned, is there a way to get kind of a list of them? I am just curious about what they can be.

由于@Eran和@Slaw的帮助，我终于可以解释一下在那里发生的输出结果

Thanks to the help of @Eran and @Slaw, I finally can explain what just happened there for the output

true
true
false
false
false

1. 由于池中不存在"Cattie& Doggie" ，因此s1.intern()会将当前对象引用放入池中并返回自身，因此 s1.intern()== s1 ;
2. "Cattie& Doggie" 现在已经在池中，因此字符串文字"Cattie& Doggie" 将仅使用池中的引用，而实际上是 s1 ，所以我们再次得到 true ;
3. new StringBuilder().toString()将在池中已有"java" 时创建一个新实例，然后调用时将返回池中的引用 s2.intern()，所以 s2.intern()！= s2 ，我们有 false ;
4. new String()也将返回一个新实例，但是当我们尝试 s3.intern()时，它将返回先前存储在池中的引用，实际上是 s1 ，所以 s3.intern()！= s3 ，我们有 false ;
5. 正如#2所讨论的那样，字符串文字"Cattie& Doggie" 将返回已经存储在池中的引用(实际上是 s1 )，因此 s3！=服装和狗狗" ，我们又遇到了 false .

1. Since "Cattie & Doggie" doesn't exist in the pool, s1.intern() will put the current object reference to the pool and return itself, so s1.intern() == s1;
2. "Cattie & Doggie" already in the pool now, so string literal "Cattie & Doggie" will just use the reference in pool which is actually s1, so again we have true;
3. new StringBuilder().toString() will create a new instance while "java" is already in the pool and then the reference in pool will be returned when calling s2.intern(), so s2.intern() != s2 and we have false;
4. new String() will also return a new instance, but when we try to s3.intern(), it will return the previously stored reference in the pool which is actualy s1 so s3.intern() != s3 and we have false;
5. As #2 already discussed, String literal "Cattie & Doggie" will return the reference already stored in the pool (which is actually s1), so s3 != "Cattie & Doggie" and we have false again.

感谢@Sunny提供技巧，以获取所有 interned 字符串.

Thanks for @Sunny to provide a trick to get all the interned strings.

推荐答案

s2.intern()仅在字符串池不存在的情况下才返回 s2 引用的实例.t在调用之前包含一个 String ，其值为"java".在执行代码之前，JDK类会内联一些 String ."java"必须是其中之一.因此， s2.intern()返回以前的被禁实例，而不是 s2 .

s2.intern() would return the instance referenced by s2 only if the String pool didn't contain a String whose value is "java" prior to that call. The JDK classes intern some Strings before your code is executed. "java" must be one of them. Therefore, s2.intern() returns the previously interned instance instead of s2.

另一方面，JDK类没有实习任何值等于"Cattie& Doggie"的 String ，因此 s1.intern()返回 s1 .

On the other hand, the JDK classes did not intern any String whose value is equal to "Cattie & Doggie", so s1.intern() returns s1.

我不知道任何预装字符串的列表.这样的列表很可能会被视为实现细节，该细节可能因不同的JDK实现和JDK版本而异，因此不应依赖.

I am not aware of any list of pre-interned Strings. Such a list will most likely be considered an implementation detail, which may vary on different JDK implementations and JDK versions, and should not be relied on.

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