String

String

• 双引号括起来的字符串创建后,长度不能发生变化
• 双引号括起来的字符串是存在于方法区的字符串常量池中
• 垃圾回收器是不会释放常量的
• 基本类型存储的是值,引用类型存储的是地址

public class Demo1 {
    public static void main(String[] args) {
        //基本类型存储的是值
        int a = 100;
        int b = 100;
        System.out.println(a==b);//true
        
        char c = 'd';
        char d = 'd';
        System.out.println(c==d);//true
        
        //引用类型存储的是地址值
        String e = new String("abc");
        String f = new  String("abc");
        System.out.println(e==f);//false
        
        String e2 = "abc";
        String f2 = "abc";
        System.out.println(e2==f2);//true
    }
}

String的常用构造方法

//1. "abc"
String a1 = "abc";
//2. new String("abc")
String a2 = new String("abc");
//3.new String(byte[])
byte[] b1 = {97,98,99};
String a3 = new String(b1);
System.out.println(a3);//abc
//4.new String(byte[],索引位置,截取长度)
String a4 = new String(b1,1,2);
System.out.println(a4);//bc
//5.new String(char[])
char[] b2 = {'我','爱','学','习'};
String a5 = new String(b2);
System.out.println(a5);//我爱学习
//6.new String(char[],索引位置,截取长度)
String a6 = new String(b2,1,3);
System.out.println(a6);//爱学习

String 的常用方法

public class Demo1 {
    public static void main(String[] args) throws UnsupportedEncodingException {
        //String的底层结构是char[]数组
        // String的常用方法
        // 1.charAt（int i） 返回所在索引的字符，从0开始
        String str = "afsdfsldk";
        char c = str.charAt(5);
        System.out.println(c); // s
        // 2.length() 返回字符串的长度(包括空格)
        int length = str.length();
        System.out.println(length);// 9
        // 3. isEmpty() 判断字符串长度是否为空，为null的话会报空指针异常
        boolean empty = str.isEmpty();// false
        System.out.println(empty);
        String str2 = null;
        try {
            boolean empty1 = str2.isEmpty();//java.lang.NullPointerException
        } catch (Exception e) {
            System.out.println("空指针异常");
        }
        // 4.compareTo(String s)从第一位开始比，如果一样，返回0，前大返回负数，前小返回正数
        // 5.compareToIgnoreCase(String s)忽略大小写，其他跟compareTo()一致
        int i = "abc".compareTo("abc");
        int i2 = "bac".compareTo("abc");
        int k = "abc".compareTo("cba");
        int[] j = new int[3];
        j[0] = i;
        j[1] = i2;
        j[2] = k;
        System.out.println(Arrays.toString(j));
        // 6.contains(String s)是否包含莫个字符串，返回boolean值
        Boolean abc = str.contains("dd");
        System.out.println(abc);
        // 7.contentEquals(new StringBuffer()) 比较String类型的字符串和StringBuffer类型的字符串的内容是否一致
        boolean d = str.contentEquals(new StringBuffer("afsdfsldk"));
        System.out.println(d);
        boolean flag = str.contentEquals(new StringBuilder("afsdfsldk"));
        System.out.println(flag);
        // 8.endsWith(String s)是否以某个字符串结尾
        //  startWith(String s)是否以某个字符串开始
        //  startWith(String s,int index)index处是否以某个字符串开始
        boolean ldk = str.endsWith("ldk");
        boolean afs = str.startsWith("afs");
        boolean sdf = str.startsWith("sdf", 2);
        System.out.println(ldk); //true
        System.out.println(afs);  //true
        System.out.println(sdf); //true
        // 9.equals(String s)和equalsIgnoreCase(String s)比较两字符串是否相等
        boolean abc1 = "abc".equals("abc");
        boolean aBc = "abc".equalsIgnoreCase("ABc");
        System.out.println(abc1); //true
        System.out.println(aBc); //true
        // 10.getBytes() 返回byte数组
        byte[] bytes = str.getBytes();
        System.out.println(Arrays.toString(bytes));// [97, 102, 115, 100, 102, 115, 108, 100, 107]
        // 11.getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)  将选中内容复制到dst数组中
        //  toCharArray() 返回一个字符串数组
        int lenth = str.length();
        char[] chars = new char[lenth];
        str.getChars(0,lenth-2,chars,2);
        char[] chars1 = str.toCharArray();
        System.out.println(Arrays.toString(chars1));// [a, f, s, d, f, s, l, d, k]
        System.out.println(Arrays.toString(chars));// [ ,  , a, f, s, d, f, s, l]
        // 12.indexOf(String s || int i || char c || int i,int i1) 返回字符串的所在索引
        //    lastIndexOf(String s || int i || char c || int i,int i1) 返回字符串的最后出现的索引
        int i3 = "abc".indexOf(97);//0
        System.out.println(i3);
        int a = "abc".indexOf('a');
        System.out.println(a); //0
        int eds = "abcd".indexOf("eds");
        System.out.println(eds); //-1
        int cd = "abcd".indexOf("cd");
        System.out.println(cd); //2
        // indexOf(int i,int i1) 从i1处开始找，是否存在i对应的字符，有则返回它的索引，无则返回-1
        int i1 = "abdcd".indexOf(97, 1);
        int c1 = "abcde".indexOf('c', 2);
        System.out.println(c1); //2
        System.out.println(i1); //-1
        int bc = "abcde".indexOf("bc", 2);
        System.out.println(bc); //-1
        int i4 = "abdabcabdaa".lastIndexOf(97);
        System.out.println(i4);

        /* 13. regionMatches((boolean ignoreCase), int toffset, String other, int ooffset, int len)
               判断 两字符串分别从指定索引开始是否相等
         */
        boolean fsd = "afsdfsdf".regionMatches(2, "fsd", 1, 2);
        System.out.println(fsd);
        /*  14.replace(String s,String s1) 将所有的s,替换成s1,如果s不存在，则不替换
               replaceFirst(String s,String s1) 替换第一个出现的s
         */
        String replace = "abcabcabc".replace("abcd", "cde");
        String replace2 = "abcabcabc".replaceAll("abc", "cde");
        String replace3 = "abcabcabc".replaceFirst("abc", "cde");
        System.out.println(replace); //abcabcabc
        System.out.println(replace2); //cdecdecde
        System.out.println(replace3); //cdeabcabc
        /*
            15.split(String s,(int i)) 用出现的第i个s分割字符串，返回字符串数组
         */
        String[] s = "dsf_sdf_sdfsd_df".split("_");
        System.out.println(Arrays.toString(s)); //[dsf, sdf, sdfsd, df]
        String[] s1 = "dsf_sdf_sdfsd_df".split("_", 2);
        System.out.println(Arrays.toString(s1));//[dsf, sdf_sdfsd_df]
        /*
        16. toUpperCase()转成大写
            toLowerCase()转成小写
         */
        String s2 = "abc".toUpperCase();//ABC
        String s3 = "ABC".toLowerCase();//abc
        System.out.println(s2);
        System.out.println(s3);
        // 17.trim()去除两边的的空格
        String trim = "   dfdsf   fsdf   ".trim();
        System.out.println(trim);
        /*18.println(Object obj)会调用String.valueOf(Object obj)方法
            Static valueOf(Object obj) 会调用对象的toString()方法
            所以，System.out.println(new Student());
            等同于System.out.println(new Student().toString());*/
        String s4 = String.valueOf(new Student());
        System.out.println(s4);  //我是Student的toString方法。。。
        System.out.println(new Student()); //我是Student的toString方法。。。
        System.out.println(new Student().toString()); //我是Student的toString方法。。。

        String format = String.format("sd-fs-df", new String[]{"sdfdsf","sdfdsf","avasds"});
        System.out.println(format);

    }
}
class Student{
    @Override
    public String toString() { return "我是Student的toString方法。。。"; }
}