Java String.intern()

Question

        String str1 = new StringBuilder("Hello").append("World").toString();
        System.out.println(str1.intern());
        System.out.println(str1 == str1.intern());

        String str2 = new StringBuilder("ja").append("va").toString();
        System.out.println(str2.intern());
        System.out.println(str2 == str2.intern());

        String str3 = new StringBuilder("hello").toString();
        System.out.println(str3.intern());
        System.out.println(str3 == str3.intern());

JDK8，这段为啥运行结果是这样的：

HelloWorld
true
java
false
hello
false