String中的intern试题解析

今天发现自己对String中的intern这个方法是我的一个盲点；对其具体的学习以后，对这些有一些理解。

String s1="123";// 常量池  
String s2=new String("123");//堆  
s2.intern();// 拷贝时发现已存在，没必要拷贝，实际无任何影响  
System.out.println(s1==s2);//false

//String s1="123";  
String s2=new String("123"); // 记得这个题吗？新建多少个对象？答案两个：常量池1个，堆上一个  
// 注意常量池其实有了123  
s2.intern();// 没有任何影响  
System.out.println("123"==s2);//false 

String s2=new String("123")+new String( "456" );  
s2.intern(); // 常量池没有123456，  
// [1.6]拷贝对象到常量池，返回新引用，但s2不变还是指向堆,s2.intern()!=s2  
// [1.7]常量池中不需要再存储一份对象了  
//拷贝引用而不是拷贝对象，也就是说s2.intern() ==s2会返回true。  
//注意前提是常量池没有这个串（或其引用），如果已经有了不会发生任何拷贝，只会返回池中引用。  
String s1="123456";// 使用常量池已有，得到引用  
System.out.println(s1==s2);// [1.6]false [1.7]true

【1.7】调用intern方法，会将字符串对象中值复制到常量池中，若常量池中已经存在该字符串，所以直接返回该字符串的引用

intern   JDK中源码 以及 注释

/** * Returns a canonical representation for the string object. * <p> * A pool of strings, initially empty, is maintained privately by the * class {@code String}. * <p> * When the intern method is invoked, if the pool already contains a * string equal to this {@code String} object as determined by * the {@link #equals(Object)} method, then the string from the pool is * returned. Otherwise, this {@code String} object is added to the * pool and a reference to this {@code String} object is returned. * <p> * It follows that for any two strings {@code s} and {@code t}, * {@code s.intern() == t.intern()} is {@code true} * if and only if {@code s.equals(t)} is {@code true}. * <p> * All literal strings and string-valued constant expressions are * interned. String literals are defined in section 3.10.5 of the * <cite>The Java&trade; Language Specification</cite>. * * @return  a string that has the same contents as this string, but is *          guaranteed to be from a pool of unique strings. */