java - 在实际生产应用程序的典型模型中，toString() 函数的真正用途是什么？弹Java

Question

在 Java 中为用户创建典型模型时提到了一段代码，我不明白为什么要使用它。

例如，用户的模型会是这样的

@Entity
@Table(name = "user")
public class User {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(unique= true, nullable = false)
private Long id;
private String firstName;
private String lastName;
private String email;
private String password;
private boolean enabled;    
private String secret;

public User() {
    super();
    this.secret = Base32.random();
    this.enabled = false;
}

// getters and setters
}

但是我在很多教程中发现的问题是我们应该另外添加：

@Override
public boolean equals(final Object obj) {
    if (this == obj) {
        return true;
    }
    if (obj == null) {
        return false;
    }
    if (getClass() != obj.getClass()) {
        return false;
    }
    final User user = (User) obj;
    if (!email.equals(user.email)) {
        return false;
    }
    return true;
}

还有 toString 函数

@Override
public String toString() {
    final StringBuilder builder = new StringBuilder();
    builder.append("User [id=").append(id).append(", firstName=").append(firstName).append(", lastName=").append(lastName).append(", email=").append(email).append(", password=").append(password).append(", enabled=").append(enabled).append("]");
    return builder.toString();
}

为什么将此代码添加到模型中？背后的原因可能是什么？是强制性的吗？

谢谢您的帮助！

标签： javaspring-mvcspring-bootspring-dataspring-data-jpa

Answer 1

toString例如用于打印输出

没有toString

class Student{  
 int rollno;  
 String name;  
 String city;  

 Student(int rollno, String name, String city){  
 this.rollno=rollno;  
 this.name=name;  
 this.city=city;  
 }  

 public static void main(String args[]){  
   Student s1=new Student(101,"Ram","Bengaluru");  
   Student s2=new Student(102,"Krishna","Chennai");  

   System.out.println(s1);
   System.out.println(s2);
 }  
} 

输出：

Student@7852e922
Student@4e25154f

在此示例中，s1打印s2位置，而不是字段值

和toString

class Student{  
 int rollno;  
 String name;  
 String city;  

 Student(int rollno, String name, String city){  
 this.rollno=rollno;  
 this.name=name;  
 this.city=city;  
 }  

 public String toString(){//overriding the toString() method  
  return rollno+" "+name+" "+city;  
 }  
 public static void main(String args[]){  
   Student s1=new Student(101,"Ram","Bengaluru");  
   Student s2=new Student(102,"Krishna","Chennai");  

   System.out.println(s1);
   System.out.println(s2);
 }  
}  

输出：

101 Ram Bengaluru
102 Krishna Chennai

在此示例中，它按预期打印了字段值。

对于equals没有的方法equals：

class Student{  
 int rollno;  
 String name;  
 String city;  

 Student(int rollno, String name, String city){  
 this.rollno=rollno;  
 this.name=name;  
 this.city=city;  
 }  

 public static void main(String args[]){  
   Student s1=new Student(101,"Ram","Bengaluru");  
   Student s2=new Student(101,"Ram","Bengaluru");  

   System.out.println(s1.equals(s2));
 }

}  

输出：假

在这个例子中，虽然 s1 和 s2 即

   Student s1=new Student(101,"Ram","Bengaluru");  
   Student s2=new Student(101,"Ram","Bengaluru");  

相同，仍然 s1.equals(s2) 的结果为假。

和equals

class Student {  
     int rollno;  
     String name;  
     String city;  

     Student(int rollno, String name, String city){  
     this.rollno=rollno;  
     this.name=name;  
     this.city=city;  
     }  

     public static void main(String args[]){  
       Student s1=new Student(101,"Ram","Bengaluru");  
       Student s2=new Student(101,"Ram","Bengaluru");  

       System.out.println(s1.equals(s2));
     }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Student other = (Student) obj;
        if (city == null) {
            if (other.city != null)
                return false;
        } else if (!city.equals(other.city))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (rollno != other.rollno)
            return false;
        return true;
    }   
}  

输出：真

在这个例子中，结果s1.equals(s2)是正确的，即真