String中的库函数

public class Main {
    public static void main1(String[] args) {
        //赋值方法
        //1.
        String a = "abcd";//直接赋值
        //2.
        String b = new String("abcde");//通过构造方法
        //3.
        char[] arr = {'a','b','c','e'};
        String c = new String(arr);//通过字符数组来构造字符串
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
        //String中的两个成员变量一个value一个hash
        //value是数组,存放的是
        System.out.println(a.length());
        String d = null;
        String e = "";
        System.out.println(e.isEmpty());//通过isEmpty()判断e是不是没有字符
        System.out.println(a.isEmpty());
    }

    public static void main2(String[] args) {
        //==判断
        String a = "abcd";
        String b = "abcd";
        String c = new String("abcde");
        String d = new String("abcde");
        System.out.println(a.equals(b));
        System.out.println(a==b);//直接赋值的时候，会存在一个字符串常量池中，再次定义一个相同的时候，会先检查常量池中有没有，如果有就指向原来的对象就行了
        System.out.println(c==d);//默认是比较地址
        System.out.println(c.equals(d));
        //忽略大小写进行比较
        String e = "ABCD";
        System.out.println(e.equalsIgnoreCase(a));
    }

    public static void main3(String[] args) {
        String a = "abcd";
        String b = "def" ;
        String c = "abcde" ;
        String d = "abcder" ;
        System.out.println(a.compareTo(b));//比较字符串的大小
        System.out.println(a.compareTo(c));
        System.out.println(a.compareTo(d));//若前面的字符都相同则当前对象比传入对象长度长几就是几
    }

    public static void main4(String[] args) {
        String a = "abcd";
        System.out.println(a.charAt(2));//获取下标为2的字符
        int index = a.indexOf('a');//通过该方法获取字符a的下标
        System.out.println(a.indexOf('a'));
        System.out.println(index);
        int index1 = a.indexOf('a',2);//从下标为2的位置开始找
        System.out.println(index1);
        System.out.println(a.indexOf("bc"));//查找bc返回b的下标
        String b = "abcdacd";
        System.out.println(b.lastIndexOf('a'));//代表从后往前找
        System.out.println(b.lastIndexOf('a', 2));//从下标为2的位置从后往前找
    }

    public static void main5(String[] args) {
        String a = String.valueOf(1234);
        int b = 1234;
        System.out.println(a.equals(b));//将其他类型转为String类型
        a += 1;
        b += 1;
        System.out.println(a+","+b);
        int c = Integer.parseInt(a);//将a转化为int类型
        System.out.println(c+1);
        String d = "ABCD";
        String a1 = "abcd";
        System.out.println(a1.toUpperCase(Locale.ROOT));//小写转大写
        System.out.println(d.toLowerCase(Locale.ROOT));//大写转小写
        System.out.println(a1);//大小写的转换不是在原字符串本身进行修改
    }

    public static void main6(String[] args) {
        String a = "abcd";
        char[] arr = a.toCharArray();//将字符串转换成字符数组
        System.out.println(Arrays.toString(arr));
        String b =new String(arr);//数组转字符串
        System.out.println(b);
    }

    public static void main7(String[] args) {
        String a = "abcd";
        System.out.println(a.replace('a', 'l'));//字符替换将a换成l
        System.out.println(a.replace("ab", "kkk"));;//字符串替换
        String b = "abababcder";
        System.out.println(b.replaceFirst("ab", "kkkk"));//将第一个ab替换掉
        System.out.println(a.replaceAll("a", "ppp"));//字符串替换不过与上面一个传入参数类型不同
    }

    public static void main8(String[] args) {
        String a = "abc&def&hij";
        String[] str = a.split("&");
        System.out.println(Arrays.toString(str));//以&分割
        String[] str1 = a.split("&",2);//最多分为两组
        System.out.println(Arrays.toString(str1));
        //特殊分割
        //split分割是根据正则表达式进行分割，部分符号需要\进行转译
        String b = "abc.def.hij";
        String[] str3 = b.split("\\.");
        System.out.println(Arrays.toString(str3));
    }

    public static void main(String[] args) {
        String a = "abcd";
        System.out.println(a.substring(2));//从指定下标开始截取
        System.out.println(a.substring(1, 2));//从指定下标开始截取到指定位置，左闭右开
        String b = "   a bcd  e ";
        System.out.println(b.trim());//去掉左右两边的空格
        
    }
}