I have created a class called student which has an equals method as shown below. My problem is this.
I create an instance of student class and add it to ArrayList, Now I want to check weather the list contains a duplicate of student object. When i try this with the below equals function its giving wrong result.
for eg.
Student stud1= new Student(101,"Student1");
Student stud5= new Student(105,"Student5");
Student stud6= new Student(105,"Student5");
list1.add(stud1);
list1.add(stud5);
System.out.println(list1.contains( new Student(105,"Student5")));// outputting false
class Student{
int sid;
String sname;
public Student(int sid,String sname){
this.sid=sid;
this.sname=sname;
}
public String toString(){
return ""+this.sid;
}
public boolean equals(Student test){
return this.sid==test.sid;
}
}
but when I replace the equals function with the one below its giving a correct result.. Why is that? Technically there is no difference right? Could you please help me as how is the JVM looking at the code..
public boolean equals(Object cond){
if(cond instanceof Student){
Student test = (Student) cond;
return test.sid==this.sid;
}
return false;
}
The first equals(Student) method doesn't override the Object#equals(Object) method. Hence with that, the list.contains(Student) method will invoke the default Object#equals() method, and which compares the references using ==. So, for two different objects, it will return false.
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