两个相同的数组，但包含方法不起作用

Question

I have a SmsClass:

public class SmsClass {

private String numberInside;
private String name;


public SmsClass( String numberInside ,String name) {
    this.numberInside = numberInside;
    this.name = name;
}

and I try to make two same ArrayList of this class:

SmsClass SmsClass3 = new SmsClass("name" , "19");
SmsClass SmsClass4 = new SmsClass("name" , "19" );

ArrayList<SmsClass> c1 = new ArrayList<>();
ArrayList<SmsClass> c2 = new ArrayList<>();

c1.add(SmsClass1);
c1.add(SmsClass2);
c2.add(SmsClass3);
c2.add(SmsClass4);

I have problem with .contains method . When I run this code :

for(int i = 0 ; i < c1.size() ; i++){
    if (c1.contains(c2.get(i))) {
        System.out.println("victory");
    }
}

I have same arrays but nothing found.

Answer 1

You should override this equals method like this in SmsClass ;

public class SmsClass {

    private String numberInside;
    private String name;


    public SmsClass(String numberInside, String name) {

        this.numberInside = numberInside;
        this.name = name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof SmsClass)) return false;

        SmsClass smsClass = (SmsClass) o;

        if (numberInside != null ? !numberInside.equals(smsClass.numberInside) : smsClass.numberInside != null)
            return false;
        return name != null ? name.equals(smsClass.name) : smsClass.name == null;
    }
}

Just an idea, you can check easily without a for loop equations of two list .

c1.removeAll(c2);
if (c1.isEmpty()) {
    System.out.println("victory!");
}

Answer 2

You must define equals in your Object.
Something like this:

public class SmsClass {

private String numberInside;
private String name;


public SmsClass( String numberInside ,String name){

    this.numberInside = numberInside;
    this.name = name;   }

public boolean equals(Object obj) {
   if(obj instanceof SmsClass ) {

            SmsClass smsObj = (SmsClass)obj;
            if(smsObj.getNumberInside() == this.getNumberInside() && 
               smsObj.getName() == this.getName())
                return true;
            }
                return false;
        }
}

Answer 3

You have not defined equals (and implicitly hashCode ), thus contains can not compare your Objects. In case you do not override those both method: Object::equals will be used (which just uses reference comparison, via == ) and hashCode (in java-8) will be computed as a pseudo-random number (based on Marsaglia XOR shift algorithm).

Answer 4

Variable names should not start with capital letter as recommendation.

Also, you added SmsClass1 , and SmsClass2 to c1 list, while adding SmsClass3 , and SmsClass4 to c2 list. Although, smsClass1 and smsClass3 have same values in their fields, they are stored on different part of memory. ArrayList.contains() method check equality according to elements' references(where they placed in memory).

SmsClass SmsClass3 = new SmsClass("name" , "19");
SmsClass SmsClass4 = new SmsClass("name" , "19" );

ArrayList<SmsClass> c1 = new ArrayList<>();
ArrayList<SmsClass> c2 = new ArrayList<>();

c1.add(SmsClass1);
c1.add(SmsClass2);
c2.add(SmsClass3);
c2.add(SmsClass4);