# String相关类

## String

* 1.String是不可变的字符数列，因为源码加了final修饰。
* 2.字符串比较

最好用equals方法（如果不明白，请看面向对象的==与equals）。

```java
package com.itcode.demo4;  

/** 
 * @Auther: 成都码到功成学员 
 * @Description: 
 * String类测试 
 */  
public class TestWrapper3 {  

    public static void main(String[] args) {  
        //  
        String a = "码到功成";  
        String b = "码到" + "功成";// 编译器自动优化拼接变成"码到功成"  
        System.out.println(a==b);// true  
        String c = "码到";  
        String d = "功成";  
        String e = c + d;  
        System.out.println(e==a);//false  
    }  
}
```

## StringBuffer和StringBuilder

概述:是可变的字符数列。StringBuffer和StringBuilder都继承了抽象类AbstractStringBuilder，源码中 ，没有加final修饰。它们的区别是：

* StringBuffer：安全，效率低。
* StringBuilder：不安全，效率高（一般使用这个）。

示例：

```java
package com.itcode.demo4;  

/** 
 * @Auther: 成都码到功成学员 
 * @Description: 
 * StringBuffer类测试 
 */  
public class TestWrapper4 {  

    public static void main(String[] args) {  
        //  
        StringBuffer sb = new StringBuffer("abc123");  
        System.out.println(sb);  
        System.out.println(Integer.toHexString(sb.hashCode()));//打印哈希值  
        sb.setCharAt(2,'S');//把下标2的值替换为S  
        System.out.println(sb);  
        System.out.println(Integer.toHexString(sb.hashCode()));  
    }  
}
```

效果：

![](https://3884904792-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LfnT30K7z1iNbyIaGVh%2F-LxVH-DsB4y0lJ8xCby5%2F-LxVHmek0O8olf1hxW2K%2Fimage.png?alt=media\&token=d61a5812-ce0d-45ac-9b86-21a648dbed16)

看哈希值都是1b6d356。

## 常用方法

StringBuffer和StirngBuilder的方法几乎一模一样。

它们有些方法和String类似，比如： indexOf(String str)、indexOf(String str, int fromIndex)、length()、charAt(int index)、substring(int start, int end)、substring(int start)。

特有方法请看代码示例：

```java
package com.itcode.demo4;  

/** 
 * @Auther: 成都码到功成学员 
 * @Description: 
 * StringBuffer和StringBuilder类测试 
 */  
public class TestWrapper4 {  

    public static void main(String[] args) {  
        //  
        StringBuilder sb = new StringBuilder();  
        // 打印26个字母  
        for (int i=0; i<26;i++){  
            sb.append((char)('a'+i));  
        }  
        System.out.println(sb);  
        System.out.println(sb.reverse());//反转  
        sb.setCharAt(1, '好');// 替换  
        System.out.println(sb);  
        sb.insert(0, "成都").insert(1,"码到功成");// 插入  
        System.out.println(sb);  
        sb.delete(5, 10);// 删除  
        System.out.println(sb);  
    }  
}
```

效果：

![](https://3884904792-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LfnT30K7z1iNbyIaGVh%2F-LxVH-DsB4y0lJ8xCby5%2F-LxVHwwVCInPBFdADEIM%2Fimage.png?alt=media\&token=bcad9910-e146-435c-b2aa-af04aeee6cda)

## 时间内存对比

可变比不可变更不占空间和耗用内存。

示例：

```java
package com.itcode.demo4;  

/** 
 * @Auther: 成都码到功成学员 
 * @Description: 
 * 测试String和StringBuilder做一样的事占用内存和时间比较 
 */  
public class TestWrapper5 {  

    public static void main(String[] args) {  
        // String  
        String s = "";  
        long num1 = Runtime.getRuntime().freeMemory();// 测试内存  
        long time1 = System.currentTimeMillis();// 获取时间毫秒值  
        for(int i=0; i<5000;i++){  
            s += i;//相当于创建10000个对象  
        }  
        long num2 = Runtime.getRuntime().freeMemory();// 测试内存  
        long time2 = System.currentTimeMillis();// 获取时间毫秒值  
        System.out.println("String所占内存："+(num1-num2));  
        System.out.println("String所花时间："+(time2-time1));  
        // String  
        StringBuilder sb = new StringBuilder();  
        long num3 = Runtime.getRuntime().freeMemory();// 测试内存  
        long time3 = System.currentTimeMillis();// 获取时间毫秒值  
        for(int i=0; i<5000;i++){  
            sb.append(i);  
        }  
        long num4 = Runtime.getRuntime().freeMemory();// 测试内存  
        long time4 = System.currentTimeMillis();// 获取时间毫秒值  
        System.out.println("StringBuilder所占内存："+(num3-num4));  
        System.out.println("StringBuilder所花时间："+(time4-time3));  
    }  
}
```

效果：

![](https://3884904792-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LfnT30K7z1iNbyIaGVh%2F-LxVH-DsB4y0lJ8xCby5%2F-LxVI37XiqP71cQG6CL_%2Fimage.png?alt=media\&token=45fbc22b-6c5d-4143-a629-283f8dffe645)


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