Java学习之==>常用字符串方法

1、定义字符串

 1 // 定义, 为初始化
 2 String str1;
 3 
 4 // 定义, 并初始化为null
 5 String str2 = null;
 6 
 7 // 定义, 并初始化为空串
 8 String str3 = "";
 9 
10 // new 空串
11 String str4 = new String();
12 
13 // new "hello"
14 String str5 = new String("hello");
15 
16 // char数组创建
17 char[] chars = {'h', 'e', 'l', 'l', 'o'};
18 String str6 = new String(chars);
19 System.out.println("str6=" + str6);
20 
21 // 在char数组中切片,从0开始,切3个长度,即:hel
22 String str7 = new String(chars, 2, 3);
23 System.out.println("str7=" + str7);
24 
25 // ASCII码组成的数组切片后再转化成字符串
26 int[] codePoints = new int[]{97, 98, 99, 100, 101};
27 String str8 = new String(codePoints, 0, 3);
28 System.out.println("str8=" + str8);
29 
30 // 将字符串转化为字节数组
31 String val = "中国";
32 String str9 = new String(val.getBytes());
33 System.out.println("str9 = " + str9);
34 
35 // 将字符串转化为字节数组并指定字符编码
36 String str10 = new String(val.getBytes("UTF-8"), StandardCharsets.UTF_8);
37 System.out.println("str10 = " + str10);

2、获取字符串的属性

 1 String str = "      ";
 2 
 3 // String str = "";   // 空串
 4 // String str = null; // 空指针
 5 // 字符串是否为空,""，若为空则返回true，若不为空则返回false
 6 boolean isEmpty = str.isEmpty();
 7 System.out.println("str.isEmpty() = " + isEmpty);
 8 
 9 // 业务中经常使用的判断操作,是不是空指针, 然后才是判断是不是空串
10 // checkpoint 检查点, 这里是一些说明
11 if (str == null || str.isEmpty()) {
12 System.out.println("这是一个空串/空指针");
13 }
14 
15 // 字符串的长度
16 int length = str.length();
17 System.out.println("length = " + length);
18 
19 // 获取字符串的hash码
20 int hashCode = str.hashCode();
21 System.out.println("hashCode = " + hashCode);

3、转化：将各种数据类型转化为字符串

 1 // 定义字符串
 2 String str = "hello world";
 3 
 4 // int & Integer 转 String
 5 Integer num1 = 1024;
 6 int num2 = num1;
 7 String intValue = String.valueOf(num2);
 8 System.out.println(intValue);
 9 
10 // long & Long 转String的操作
11 Long num3 = 5689087678L;
12 long num4 = num3;
13 String longValue = String.valueOf(num4);
14 System.out.println(longValue);
15 
16 // float(Float) & double(Double) 转String,
17 String.valueOf(10.24F);
18 String.valueOf(10.24D);
19 
20 // boolean类型转化成String
21 String.valueOf(false);
22 
23 // 字节转化为String
24 String.valueOf('a');
25 
26 // 对象转化成String
27 String.valueOf(new byte[]{1, 2, 3, 4});
28 String.valueOf(new char[]{'a', 'b', 'c', 'd'});

4、分割与连接

 1 String str = "hello,world,hi,thank,you";
 2 // 分割
 3 String[] strarray = str.split(",");
 4 
 5 // 数组遍历
 6 for (String s : strarray) {
 7   System.out.println(s);
 8 }
 9 
10 System.out.println("\n-----我是分割线------\n");
11 
12 String[] strarray2 = str.split(",", 3);
13 for (String s : strarray2) {
14   System.out.println(s);
15 }
16 
17 System.out.println("\n-----我是分割线------\n");
18 
19 // 连接
20 String joinResult1 = String.join("+", strarray);
21 System.out.println("joinResult1 = " + joinResult1);

5、截取

 1 String str = "123456789";
 2 
 3 // 从beginIndex开始截取,一直截取到最后
 4 String res1 = str.substring(3);
 5 System.out.println("res1 = " + res1);
 6 
 7 System.out.println("\n-----我是分割线------\n");
 8 
 9 // 从beginIndex开始截取一直截取到endIndex
10 String res2 = str.substring(3, 6);
11 System.out.println("res2 = " + res2);
12 
13 System.out.println("\n-----我是分割线------\n");
14 
15 String originStr = "  \nabcdefg\n    ";
16 System.out.println("origin:[" + originStr + "]");
17 
18 // 去掉字符串两边的空格和换行符
19 originStr = originStr.trim();
20 System.out.println("origin:[" + originStr + "]");

6、定位

 1 String str = "hello,love,how are you, love,thanks,";
 2 
 3 // 从前向后查找
 4 int index = str.indexOf("love");
 5 System.out.println("love first index = " + index);
 6 
 7 index = str.indexOf("love", 7);
 8 System.out.println("love jump index = " + index);
 9 
10 // 循环定位所有的字符串
11 String[] strarray = str.split(",");
12 for (String s : strarray) {
13   int i = str.indexOf(s);
14   System.out.println(s + ":" + i);
15 }
16 
17 index = str.indexOf('e');
18 System.out.println("char index = " + index);
19 
20 // 从后向前查找
21 index = str.lastIndexOf("love");
22 System.out.println("last index = " + index);
23 
24 // 找不着，返回-1
25 index = str.indexOf("yes");
26 System.out.println("none exists index = " + index);
27 
28 // 根据下标返回字符
29 char ch = str.charAt(0);
30 System.out.println("ch = " + ch);
31 ch = str.charAt(1);
32 System.out.println("ch = " + ch);

7、判断：值相等,地址相等,前缀相等,后缀相等,包含

String str1 = "hello";
String str2 = "hi";
String str3 = "hello";
String str4 = "HELLO";

// 判断字符串的值是否相等，不相等,返回false,相等则返回的true
boolean isEq = str1.equals(str2);
System.out.println("1:" + isEq);
isEq = str1.equals(str3);
System.out.println("2:" + isEq);

// 判断内存地址是否相等
isEq = (str1 == str3);
System.out.println("3:" + isEq);
System.out.println(System.identityHashCode(str1));
System.out.println(System.identityHashCode(str3));

// 忽略大小写的 相等判断
isEq = str1.equalsIgnoreCase(str4);
System.out.println("4:" + isEq);
isEq = str1.equals(str4);
System.out.println("5:" + isEq);

// 起始前缀匹配判断 ,210_ 支付宝,230_ 微信,
String orderId1 = "210_100012342324324";
String orderId2 = "230_100012342324324";
boolean isStartWith1 = orderId1.startsWith("210_");
boolean isStartWith2 = orderId2.startsWith("210_");
System.out.println("isStartWith:" + isStartWith1);
System.out.println("isStartWith:" + isStartWith2);

// 末尾后缀匹配判断
final boolean isEndWith = orderId1.endsWith("4324");
System.out.println("isEndWith = " + isEndWith);

// 包含判断,如果包含子串返回true, 不包含返回false
String str = "hello world,hi";
final boolean isContains = str.contains("hi");
System.out.println("isContains = " + isContains);

8、转换&&替换

// 将所有字符转换为大写
String str = "hello";
String upperCase = str.toUpperCase();
System.out.println("upperCase = " + upperCase);

// 将所有字符转换为小写
String str2 = "HELLO";
final String lowerCase = str2.toLowerCase();
System.out.println("lowerCase = " + lowerCase);

// 将字符串转换为字符数组
String str3 = "abcde";
char[] chars = str3.toCharArray();
for (char ch : chars) {
  System.out.println(ch);
}

// 替换目标字符串
"hello world".replace("hello", "hi");
"hello world,hello java".replaceAll("hello", "hi");
"hello world,hello java".replaceFirst("hello", "hi");

9、面试点

1、String str = new String(“Hello”) 这段代码创建了几个对象
两个，字符串“Hello”存在常量池中，由这个字符串创建的一个新对象（存在堆内存当中）赋给了str

2、
  public void interviewPoint1() {
    String str1 = "hello";
    String str2 = new String("hello");

    System.out.println(str1 == str2); // false
    System.out.println(str1.equals(str2)); // true
  }

3、
  public void interviewPoint2() {
    String str1 = "hello";
    String str2 = "hello";

    System.out.println(str1 == str2); // true
    System.out.println(str1.equals(str2)); // true
  }

4、
  public void interviewPoint3() {
    String str1 = "a" + "b" + "c";
    String str2 = "abc";

    System.out.println(str1 == str2); // true
    System.out.println(str1.equals(str2)); // true
  }

5、
  public void interviewPoint4() {
    String str1 = "ab";
    String str2 = "abc";
    String str3 = str1 + "c";

    System.out.println(str2 == str3); // false
    System.out.println(str2.equals(str3)); // true
  }

10、谈谈String, StringBuilder, StringBuffer的区别

 

来源：https://www.cnblogs.com/L-Test/p/11350756.html