JavaSE__Integer 的默认初始值

Integer 如没有赋初值会为null，会引发空指针异常，示例代码如下

package com.login.bean;

public class UserObject {
private Integer user_id;
private String user_name;
private String user_pass;

{
user_name = "";
user_pass = "";
}

public int getUser_id() {
return user_id;
}

public void setUser_id(int user_id) {
this.user_id = user_id;
}

public String getUser_name() {
return user_name;
}

public void setUser_name(String user_name) {
this.user_name = user_name;
}

public String getUser_pass() {
return user_pass;
}

public void setUser_pass(String user_pass) {
this.user_pass = user_pass;
}

public UserObject(int user_id, String user_name, String user_pass) {
super();
this.user_id = user_id;
this.user_name = user_name;
this.user_pass = user_pass;
}

public UserObject(String user_name, String user_pass) {
super();
this.user_name = user_name;
this.user_pass = user_pass;
}

public UserObject(String user_name) {
super();
this.user_name = user_name;
}

public UserObject() {

}

public static void main(String[] args) {
UserObject testObject = new UserObject();
System.out.println(testObject.getUser_id() == 0);
System.out.println(testObject.getUser_name() == "");
System.out.println(testObject.getUser_pass() == "");
}
}


注意这里user_id 没有赋予初始值，会引发异常
运行后错误如下：