执行以下代码
String a1=new String("abc");
String a2=new String("abc");
System.out.println(a1==a2);
System.out.println(a1==a2.intern());
System.out.println("abc"==a2.intern());
System.out.println(a1.intern()==a2.intern());
返回
false
false
true
true
new String("abc") 方法是创建一个String对象， 同时引用常量池里的"abc", a1,a2分别是两块内存空间，a1!=a2; 而常量"abc"的引用是和a1,a2都不一样的, a1!=a2.intern(); "abc"是常量池的引用,和a2.intern()是指向同一个常量，所以"abc"==a2.intern()
JDK源码里有这么段说明
/**
* 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™ 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.
*/
public native String intern();
再来看看下面这段代码
String a1="abc";
String a2="abc";
System.out.println(a1==a2);
System.out.println(a1==a2.intern());
System.out.println("abc"==a2.intern());
System.out.println(a1.intern()==a2.intern());
全部返回true, a1,a2都是指向"abc"常量池
再来看看下面这段
String a1="abc";
String a2=new String("abc");
System.out.println(a1==a2);
System.out.println(a1==a2.intern());
System.out.println("abc"==a2.intern());
System.out.println(a1.intern()==a2.intern());
返回
false
true
true
true
PS: 相同字符串的intern()返回大部分情况是一样的， 因为存在调用了intern()方法后，该字符串被回收了，那么再次调用intern()会把相同的字符串加入到常量池里，但引用位置已经改变