Java有哪些操作字符串的类?区别在哪?

public class StringTest {
    public static void main(String[] args) {
        String s1 = "abc";
        String s2 = "abc";
        String s3 = new String("abc");	
        System.out.println(s1 == s2);	// true
        System.out.println(s2 == s3);	// false
        s2 = "abc" + "def";
        System.out.println(s1 == s2);	// false
        String s4 = "abcdef";
        System.out.println(s4 == s2);	// true
    }
}

public class StringTest2 {
    public static void main(String[] args) {
        StringBuffer sb1 = new StringBuffer("abc");
        StringBuilder sb2 = new StringBuilder("abc");
        System.out.println(sb1.hashCode());	// 460141958
        System.out.println(sb2.hashCode());	// 1163157884
        sb1.append("def");
        sb2.append("def");
        System.out.println(sb1.hashCode()); // 460141958
        System.out.println(sb2.hashCode()); // 1163157884
    }
}

   @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

  @Override
    public synchronized StringBuffer append(Object obj) {
        toStringCache = null;
        super.append(String.valueOf(obj));
        return this;
    }

 public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        // 查看空间是否足够
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

 private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        // 如果空间不够就重新开辟一块新的空间
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

    public static char[] copyOf(char[] original, int newLength) {
        char[] copy = new char[newLength];
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }