String类

/**
测试String类
*/
public class TestString02 {
  public static void main(String[] args) {
    String str1 = "aaabbb";  //不可再改
    
    String str2 = str1.substring(2,5);
    
    System.out.println(str2);   //abb
    
    String str3 = "hello" + " java";    //相当于str1 = "hello java"
    String str4 = "hello java";
    System.out.println(str3 == str4);   //true
    
    String str5 = "hello";
    String str6 = " java";
    //编译时不知道变量存储的是什么，所以没有办法在编译的时候优化
    String str7 = str5 + str6;
    System.out.println(str4 == str7);   //false
 
    System.out.println(str4.equals(str7));  //true,字符串比较要用equals，不要用==
    }
  }