字符串操作常用方法和注意事项

字符串操作常用方法和注意事项

    //在jvm字符串常量池中
    String s = "";
    //在字符串常量池中
    String s1 = "hello";
    //在堆中
    String s2 = new String("hello");
    //在java6之后引入intern（）方法去重复（字符串常量池和堆中）
    String s3 = s2.intern();
    System.out.println(s3);//hello

    //假如不重复
    String s5 = "hel";
    String s4 = s5.intern();
    System.out.println(s4);//hel

    String s6 = "hellohello";
    String s7 = "worldworld";
    String s8 = "hellohelloworldworld";
    String s9 = new String("hellohelloworldworld");
    System.out.println(s8 == s9);//false
    System.out.println(s8.equals(s9));//true

    String s10 = s6 + s7;
    System.out.println(s10 == s9);//false

    /**
     * s11的操作是最后返回一个引用
     */
    String s11 = s6 + "worldworld";
    System.out.println(s11 == s8);//false
    System.out.println(s11 == s9);//false

    /**
     * 为了节约空间s12和s13指向同一个内存地址
     */
    String s12 = "hello.";
    String s13 = "hello.";
    System.out.println(s12 == s13);//true

    /**
     * String操作
     * 长度：采用的是utf-16的编码
     * 一个是代码单元一个是代码点
     */
    String s14 = "he？";
    /**
     *代码单元：
     * utf - 16：0041
     * utf - 8 ：41
     */

    System.out.println(s14.length());
    //代码点个数

    System.out.println(s14.codePointCount(0, s14.length()));
    /**
     * String在拼接中会出现大量的对象
     * Buffer安全是通过syn锁 ，buffer和bulider都是继承抽象的bulider
     * 底层都是通过char数组（9）之前，9之后是byte数组
     * StringBuffer和StringBulider默认初始的容量是16
     * 能评估先评估字符串的大小（setLength）避免扩容
     * 扩容是，删除原来数组，创建新的数组，arraycopy
     */
    StringBuffer sb = new StringBuffer();
    sb.append("hello");
    sb.append("world");
    sb.append("!");
    System.out.println(sb);//helloworld!
    System.out.println(sb.toString()); //helloworld!

    StringBuilder sb1 = new StringBuilder();
    sb1.append("hello");
    sb1.append("world");
    sb1.append("!");
    System.out.println(sb1);//helloworld!
    System.out.println(sb1.toString());//hellowprld!

    System.out.println(sb1.length()); //11
    sb1.setLength(sb1.length() - 1);
    System.out.println(sb1.toString());//helloworld
    sb1.setLength(sb1.length() + 10);
    System.out.println(sb1);//helloworld

    /**
     * jvm的智能优化
     */
    String s15 = "hello" + "," + "world";
    String s16 = "hello,world";
    System.out.println(s15 == s16);//优化后在同一位置true

    String s17 = "hello";
    String s18 = s17 + "world";

    StringBuilder sb3 = new StringBuilder();
    sb3.append(s17);
    sb.append("world");

    System.out.println(sb3.toString() == s18);//优化后不在同一个位置

    /**
     * 最快的
     */
    long time = System.currentTimeMillis();
    StringBuilder sb4 = new StringBuilder();
    for (int i = 0; i < 100000; i++) {
        sb4.append(i);
    }
    System.out.println(System.currentTimeMillis() - time);


    /*
    去掉空格不使用trim方法
     */
    String s19 = " hdihsi sdjsod ";
    String s20 = s19.trim();
    System.out.println(s20);//hdihsi sdjsod只能去掉首尾空格

    String s21 = " helo shidhs hdis hi ";
    String s22 = s21.replace(" ", "");
    System.out.println(s22);//heloshidhshdishi去掉首尾和中间的空格

    System.out.println(trimAll(s22));

    /*
    字符串反转
    格式规范的快捷键ctrl+alt+l
     */
    String s23 = "--123 321 ,0 ";
    System.out.println(reverse(s23));
    StringBuilder sb6 = new StringBuilder();
    sb6.append(s23);
    System.out.println(sb6.reverse().toString());

}

private static String reverse(String s) {
    StringBuilder sb = new StringBuilder();
    char[] array = s.toCharArray();
    for (int i = array.length-1 ; i >= 0; i--){
      sb.append(array[i]);
    }
    return sb.toString();
}

private static String trimAll(String s) {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < s.length(); i++) {
        char temp = s.charAt(i);
        if (temp == ' ') {
            continue;
        } else {
            sb.append(temp);
        }
    }
    return sb.toString();
}

}