理解String的intern()方法

API文档中的介绍：

intern

public String intern()

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.

All literal strings and string-valued constant expressions are interned. String literals are defined in section 3.10.5 of the The Java™ Language Specification.

Returns:

a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings.

意思就是说：

该方法返回一个规范化表示的string对象。

一个字符串常量池，初始化为空，且被String类私有地维护。

当intern()被调用时，如果常量池中存在和该String对象equal的对象（由equals方法决定），则返回常量池中已有的String对象，若不存在，则将该对象加入常量池，并返回该对象的引用。

代码验证如下：

public static void main(String[] args){
        String a="abc";
        String b=new String("abc");
        String c=b.intern();
        System.out.println(b==a);
        System.out.println(b==c);
        System.out.println(c==a);
    }

输出结果为：

false
false
true