• java.lang.String

/**
     * 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.
     */
    public native String intern();

• intern方法用来返回常量池中的某字符串，如果常量池中已经存在该字符串，则直接返回常量池中该对象的引用。否则，则常量池中加入该对象，然后返回引用。

String设计成不可变的原因

• 字符串常量池的需要。字符串常量池的诞生可以提高效率，减少内存分配。String的不可变，常量池可以很容易的被管理和优化。
• 安全性考虑。字符串使用的场景很多，设计成不可变的可以有效的防止字符串被有意或无意的被篡改。
• 作为HashMap、HashTable等hash类型key的必要。因为String被设计成不可变的，JVM底层很容易在缓存String对象的时候缓存其hsahCode，这样在执行效率上会大大提升。

创建字符串

• 直接使用双引号创建字符串
  • 判断这个常量是否存在于常量池
    • 如果存在，判断这个常量是存在的引用还是常量
      • 如果是引用，则返回引用地址指向的堆空间对象
      • 如果是常量，则直接返回常量池常量
    • 如果不存在
      • 在常量池中先创建该常量，并返回此常量

        String s1 = "hello";  // 在常量池中创建常量
        String s2 = "hello";  // 直接返回已经存在的常量

        String s1 = "hello";
        String s2 = "hello";
        System.out.println(s1 == s2);        // true
        System.out.println(s1.equals(s2));    // true

        String s1 = new String("hello");  // 在堆上创建字符串对象
        s1.intern();    // 在常量池中创建对象的引用
        String s2 = "hello";  // 常量池中存在该常量，直接返回该引用指向的堆空间对象

        String s1 = new String("hello").intern();
        String s2 = "hello";
        System.out.println(s1 == s2);      // true
        System.out.println(s1.equals(s2));      // true

• new String();
  • 首先在堆上创建对象
  • 然后判断常量池上是否存在字符串的字面量
    • 如果不存在，在常量池中创建常量
    • 如果存在，不做任何操作

        String s1 = new String("hello");
        String s2 = new String("hello");
        System.out.println(s1 == s2);      // false
        System.out.println(s1.equals(s2));    // true

        String s1 = new String("hello").intern();
        String s2 = new String("hello").intern();
        System.out.println(s1 == s2);    // true
        System.out.println(s1.equals(s2));    // true

        String s1 = new String("hello").intern();
        String s2 = "hello";
        System.out.println(s1 == s2);    // true
        System.out.println(s1.equals(s2));  // true

• String.valueOf();

    /**
     * Returns the string representation of the {@code Object} argument.
     *
     * @param   obj   an {@code Object}.
     * @return  if the argument is {@code null}, then a string equal to
     *          {@code "null"}; otherwise, the value of
     *          {@code obj.toString()} is returned.
     * @see     java.lang.Object#toString()
     */
    public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

        String s1 = String.valueOf("hello");
        String s2 = "hello";
        System.out.println(s1 == s2);    // true
        System.out.println(s1.equals(s2));    // true

        String s1 = String.valueOf("hello");
        String s2 = String.valueOf("hello");
        System.out.println(s1 == s2);    // true
        System.out.println(s1.equals(s2));    // true

        String s1 = String.valueOf("hello");
        String s2 = new String("hello").intern();
        System.out.println(s1 == s2);    // true
        System.out.println(s1.equals(s2));    // truee