淺談java中String類的intern方法

先說String，它是一個finally類，即不可被繼承，其方法默認也是finally不可被重寫。查看源碼可知intern是一個內部方法，下面是jdk11裏的原文：

/**
     * 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.
     * @jls 3.10.5 String Literals
     */
    public native String intern();

這是一個native方法，由String類私有的維護這個String池，它保證池內每個數據都只有一份。若有新的String對象賦值會先在String池中查找該值是否存在，存在則直接將引用指向該值，不存在則創建新的值。注意，用new創建字符串會創建兩個數據，一個在堆裏創建，一個是在String池，創建後引用會指向堆中數據，調用intern方法後指向String池數據。

具體看下面的例子

        String a = "asdf";
        String b = "as";
        String c = "df";
        String d = b+c;
        String e = "as"+"df";

        String f = new String("asdf");
        String g = new String("as")+new String("df");
        String h = "as" + new String("df");
        String i = new String("as") + "df";

        String aa = "1111";
        String bb = new String("11") + new String("11");

        System.out.println("================");       //   ================
        System.out.println(a == d);                   //   false   引用地址不同
        System.out.println(a == e);                   //   true    引用地址相同，String池只會維護一份數據，數據相同時直接修改引用不會新建相同的數據
        System.out.println(d == e);                   //   false
        System.out.println(a == d.intern());          //   true    intern()指向String池中的值
        System.out.println("================");       //   ================
        System.out.println(a == f);                   //   false   new在堆中創建，等號賦值在String池中
        System.out.println(a == g);                   //   false
        System.out.println(a == h);                   //   false
        System.out.println(a == i);                   //   false
        System.out.println(a == f.intern());          //   true    調用intern後引用就指向了String池中的數據
        System.out.println(a == g.intern());          //   true
        System.out.println(a == h.intern());          //   true
        System.out.println(a == i.intern());          //   true
        System.out.println("================");       //   ================
        System.out.println(f == g);                   //   false
        System.out.println(f == d);                   //   false
        System.out.println(f.intern() ==  d);         //   false
        System.out.println(f == d.intern());          //   false
        System.out.println(f.intern() == d.intern()); //   true
        System.out.println(aa == bb);                 //   false
        System.out.println(aa == bb.intern());        //   true
        System.out.println("================");       //   ================