字符串-StringBuilder

1、StringBuilder概述

1.1、什么是StringBuilder

我们如果对字符串进行拼接操作，每次拼接，都会构建一个新的String对象，既耗时，又浪费空间。而StringBuilder就可以解决这个问题。

1.2、StringBuilder与String区别

String的内容是固定的。StringBuilder的内容是可变的

1.3、StringBuilder构造方法

StringBuilder() 构造一个没有字符的字符串构建器，初始容量为16个字符。 StringBuilder(int capacity) 构造一个没有字符的字符串构建器，以及由 capacity参数指定的初始容量。
StringBuilder(CharSequence seq) 构造一个字符串构建器，其中包含与指定的 CharSequence相同的字符。
StringBuilder(String str) 构造一个初始化为指定字符串内容的字符串构建器。

2、StringBulider常用方法

2.1、常用方法1

public int capacity():返回当前容量,理论值

public int length():返回长度（字符数）,实际值

public class StringBuilderDemo01 {
    public static void main(String[] args) {
        //创建对象
        StringBuilder builder = new StringBuilder();
        System.out.println("builder:" + builder);
        System.out.println("builder.capacity():"+ builder.capacity());
        System.out.println("builder.length():"+ builder.length());
        System.out.println("---------------------");
        //创建对象
        StringBuilder builder1 = new StringBuilder("HelloWorld");
        System.out.println("builder1:" + builder1);
        System.out.println("builder1.capacity():"+ builder1.capacity());
        System.out.println("builder1.length():"+ builder1.length());
    }
}

运行结果：

2.2、常用方法2

添加功能：public StringBuilder append(任意类型):添加数据，并返回自身对象

反转功能：public StringBuilder reverse()

public class StringBuilderDemo02 {
    public static void main(String[] args) {
        //创建对象
        StringBuilder builder = new StringBuilder();
        //public StringBuilder append(任意类型)
        StringBuilder builder2 = builder.append("hello");
        System.out.println("builder:" + builder);
        System.out.println("builder2:" + builder2);

        System.out.println(builder == builder2); //true

        builder.append("hello");
        builder.append("world");
        builder.append(true);
        builder.append(100);

        //链式编程
//builder.append("hello").append("world").append(true).append(100);
        System.out.println("builder:" + builder);

        //public StringBuilder reverse()
        builder.reverse();

        System.out.println("builder:" + builder);
    }
}

运行结果：

2.3、与String的相互转换

StringBuilder --> String

public String toString():通过toString()就可以实现把StringBuilder转成String

String --> StringBuilder

StringBuilder(String str):通过构造方法就可以实现把String转成StringBuilder

public class StringBuilderDemo03 {

    public static void main(String[] args) {
        //StringBuilder -- String

        StringBuilder builder = new StringBuilder();
        builder.append("Hello").append("World");

        String s = builder.toString();
        System.out.println(s);


        //String -- StringBuilder
        String s1 = "HelloWorld";
        StringBuilder builder1 = new StringBuilder(s1);
        System.out.println(builder1);
    }
}

运行结果：