易出错的代码整理

Posted 2020-09-27

 1 public class HelloA {
 2     public HelloA() {
 3         System.out.println("HelloA");
 4     }
 5 
 6     {
 7         System.out.println("lam A class");
 8     }
 9     static {
10         System.out.println("static A");
11     }
12 
13 }

 1 public class HelloB extends HelloA {
 2     public HelloB() {
 3         System.out.println("HelloB");
 4     }
 5 
 6     {
 7         System.out.println("lam B class");
 8     }
 9     static {
10         System.out.println("static B");
11     }
12 
13     public static void main(String[] args) {
14         new HelloB();
15     }
16 }

执行结果:
1 static A
2 static B
3 lam A class
4 HelloA
5 lam B class
6 HelloB

 

1 public class Test {
2     public static void main(String[] args) {
3         String str1 = "Hello";
4         String str2 = "He" + new String("llo");
5         System.out.println(str1 == str2);
6         str2 = "He" + "llo";
7         System.out.println(str1 == str2);
8     }
9 }

比较结果:

false

true