成神之路-基础篇4-String

一、String相关类层次

1. String

2. StringBuffer

3. StringBuilder

StringBuffer.jpg

二、常用方法

1. 构造

String() ：String的字符数组为final，所以不建议这么构造
String(byte[] bytes)
String(byte[], int offset, int length)
String(byte[], int offset, int length, String charsetName)
String(byte[], int offset, int length, Charset charset)
String(String original)
String(StringBuffer buffer)
String(StringBuilder builder)

2. 字符串链接

操作符+
concat
append

public class TestString {
    public static void main(String[] args) {
        String s1 = new String("hello ");
        String s2 = new String("damon!");
        System.out.println(s1 + s2);
        System.out.println(s1.concat(s2));
        StringBuilder sbuilder = new StringBuilder("hello ");
        sbuilder.append("stefan!");
        System.out.println(sbuilder);
        StringBuffer sbuf = new StringBuffer("hello ");
        sbuf.append("Elena!");
        System.out.println(sbuf);
    }
}

3. 子字符串：为啥StringBuilder没有这两个方法

public class TestString {
    public static void main(String[] args) {
        String s1 = new String("hello ");
        System.out.println(s1.substring(1));
        System.out.println(s1.substring(1,4));
    }
}

4. 查找

public class TestString {
    public static void main(String[] args) {
        String s1 = new String("hello ");
        System.out.println(s1.indexOf('l'));
        System.out.println(s1.indexOf('l', 3));
        System.out.println(s1.lastIndexOf('l'));
        System.out.println(s1.lastIndexOf('l', 2));
    }
}

5. 转换

public class TestString {
    public static void main(String[] args) {
        String s1 = String.valueOf(1);
        String s2 = String.valueOf('a');
        String s3 = String.valueOf(3.14);
        int i = Integer.valueOf("3");
        double d = Double.valueOf("3.14");
        System.out.println(s1);
        System.out.println(s2);
        System.out.println(s3);
        System.out.println(i);
        System.out.println(d);
    }
}

三、使用注意点及最佳实践

1. for循环中慎重用String +

public class TestString {
    public static void main(String[] args) {
        String s1 = new String("hello");
        for (int i = 0; i < 10000; i++) {
            s1 = s1 + i;
        }
    }
}

反编译

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/lang/String
       3: dup
       4: ldc           #3                  // String hello
       6: invokespecial #4                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
       9: astore_1
      10: iconst_0
      11: istore_2
      12: iload_2
      13: sipush        10000
      16: if_icmpge     44
      19: new           #5                  // class java/lang/StringBuilder
      22: dup
      23: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V
      26: aload_1
      27: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      30: iload_2
      31: invokevirtual #8                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      34: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      37: astore_1
      38: iinc          2, 1
      41: goto          12
      44: return

for循环中，会新建StringBuilder对象，调用append方法，每次循环都调用一次，性能差

2. String、StringBuffer、StringBuilder效率对比

public class TestString {
    public static void main(String[] args) {
        String s1 = new String("hello");
        long t1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            s1 = s1 + i;
        }
        long t2 = System.currentTimeMillis();
        System.out.println("String:" + (t2 - t1));

        StringBuilder builder = new StringBuilder("hello");
        long t3 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            builder.append(i);
        }
        long t4 = System.currentTimeMillis();
        System.out.println("StringBuilder:" + (t4 - t3));

        StringBuffer buffer = new StringBuffer("hello");
        long t5 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            buffer.append(i);
        }
        long t6 = System.currentTimeMillis();
        System.out.println("StringBuffer:" + (t6 - t5));
    }
}

结果

String:331
StringBuilder:0
StringBuffer:1

String在for循环中+会转化为创建StringBuilder对象的append方法，因为每次都创建对象，所以很慢
StringBuilder相比于StringBuffer相比不需要同步，所以较快

四、其他注意点

1. 语法糖而非操作符重载

反编译后知道，String的+实际会被编译器转为StringBuilder.append()

参考
https://hollischuang.github.io/toBeTopJavaer/#/basics/java-basic/string-concat

最后编辑于：2020.06.13 16:10:03