Interview Questions and Answers for 'Equals' | Search Interview Question

Interview Questions and Answers for 'Equals' - 23 question(s) found - Order By Newest

Very frequently asked. Among first few questions in almost all interviews. Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys,Capgemini etc based on multiple feedback ) and Epam Systems

Q1. Difference between == and .equals() ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object.

Sample code:

String x = new String("str");
String y = new String("str");

System.out.println(x == y); // prints false
System.out.println(x.equals(y)); // prints true

Sample Code for equals

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Q2. What will be the output of following Code ?

class BuggyBread {
   public static void main(String[] args) {
      String s2 = "I am unique!";
      String s5 = "I am unique!";

      System.out.println(s2 == s5);
   }
}

Core Java

Ans. true, due to String Pool, both will point to a same String object.

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Q3. Can we compare Integers by using equals() in Java ?

Core Java

Ans. Yes for the Wrapper class Integer but not for the primitive int.

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Very Frequently asked. Favorite question in walkins and telephonic interviews. Usually among first few questions. Asked in different variants. Must know for intermediate and expert professionals.Among Top 10 frequently asked questions.

Q4. What is rule regarding overriding equals and hashCode method ?

Core Java

Ans. A Class must override the hashCode method if its overriding the equals method.

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Q5. Why is null == null returns true ?

Core Java

Ans. Any reference in java that doesn't point to any object , gets assigned null i.e is a reference to null. Two object references in java are treated equal if they point to the same memory. That's Why null == null results true.

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Q6. What will be the output ?Integer a = 10, b =10;Integer c = 10, d = 1000;System.out.println(a == b);System.out.println(c ==d);

Core Java

Ans. truefalse

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Q7. What will the following code print ?

Integer a = 100, b =100;
Integer c = 1000, d = 1000;
System.out.println(a == b);
System.out.println(c ==d);

Core Java

Ans. false
false

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Q8. What is wrong with the following if statement ?

if(x==y && x.equals(y) {
}

Core Java

Ans. x==y means that both references have same type and are pointing to same memory location and hence would always mean that they have same value.

x.equals(y) is not required in this case.

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Q9. What is wrong with the following if statement ?

if(x==y || x.equals(y) {
}

Core Java

Ans. if x==y turns out to be true x.equals(y) will be true too. If x.equals(y) could be true even if x==y is true or not.

So the only possible outcomes are

1 || 1 = 1
0 || 1 = 1
0 || 0 = 0

i.e the outcome of x.equals(y)

check for x==y is not required in this if statement.

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Q10. What is the difference between = and == in Java ?

Core Java

Ans. = is the assignment operator that assigns the result of the expression on the right to the variable on the left, whereas

== is the operator to check object equality to see if the reference on left and right are pointing to the same object. For primitive types, its used to check if both variables holds the same value.

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Q11. How is == operator different for objects and primitive types ?

Core Java

Ans. For objects or references, == operator check if the reference on left and right points to the same object.

For primitive types or variables, == operator check if the variable on left and right holds the same value.

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Q12. In Java, if object A contains same the same data of object B, are they equal?

Core Java

Ans. it depends on the implementation of equals method of the respective class. If no definition is provided and it uses the default definition of the object class, two references are equal only if they point to the same object.

We can have such an equality defined for a particular class objects if we provide appropriate implementation of equals method comparing all those fields.

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Q13. What will be the output of following code

Integer x = 1;
Integer y = 2;
System.out.println(x == y);

What if you change 1 to "1" and Integer to String?

Core Java

Ans. It will print "true" with integers as well as strings. The reason is "Integer constant pool" and "String pool"

String pool maintains pool of string literals. When a string literal is used for the first time, a new string object is created and is added to the pool. Upon it's subsequent usage , the reference for the same object is returned. Similarly java uses integer constant pool.

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Q14. What is equals method in Java ?

Core Java

Ans. equals is the method of Object class that is overridden by the classes to specify object equality criteria.

As every class extends Object class, the default implementation of equals is carried to them. Default implementation specified in the Object class is that two objects are treated equal if they are same.

i.e

Object x = new Object();
Object y = new Object();
x.equals(y); // false
x=y;
x.equals(y); // true

i.e x.equals(y) if only x==y

Now every class has the option to specify their object equality by overriding equals method. For example - String class has implemented in a manner if the string value contained in them is exactly same.

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Q15. Which of the following is valid greater than and equal to operator in Java ?

>=
=>

Core Java

Ans. >=

=> will result in error.

=> somewhat looks like lambda operator "->"

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Q16. What will be the output of following code

class TestMain {
public static void main(String[] args) {
String s1 = "ravi";
String s2 = new String("ravi");
Integer i = 10;
Integer a1 = new Integer("10");
System.out.println(s1 == s1);
System.out.println(i == a1);
}
}

Core Java

Ans. false
false

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Q17. What is a reflexive equals contract ?

Core Java

Ans. An equal contract is said to be reflexive is equality between 2 objects always returns to be true.

For example -

Object o1;
Object o2;

o1.equals(o2); // returns true
o2.equals(o1); //returns true

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Q18. Explain Transitive equals contract.

Core Java

Ans. The equals contract is said to be transitive if

obj1.equals(obj2); and obj2(.equals(obj3)

then obj1.equals(obj3) also returns true.

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Q19. why is "".equals(str); safer than str.equals("")?

Core Java

Ans. str.equals("") this statement will throw NullPointerException if str is null where as "".equals(str) works fine even if str is null

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Q20. Difference between equals and compareTo in Java ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality. x.equals(y) means the references x and y are holding objects that are equal.

The compareTo() method is used for comparing two objects in Java. It is usually defined for the classes whose objects needs to be ordered through Comparable interface or need to be part of an ordered collection like TreeSet or TreeMap.

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Q21. Difference between equals and hashcode in java ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality. x.equals(y) means the references x and y are holding objects that are equal with the equality defined by the definition of equals method.

Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not.

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Q22. Difference between equals and equalsignorecase in Java ?

Core Java

Ans. "equals" method is the method of object class that needs to be overridden to check object equality. This is not specific to any class like String.

equalsignorecase is the method of String class that provides a definition that ignores the case of characters during comparison.

The only difference between them in String class is that the equals() methods considers the case while equalsIgnoreCase() methods ignores the case during comparison.

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Q23. What is the use of defining equals , compareTo and hashcode methods in a class ? Where are they used ?

Core Java

Ans. equals, compareTo and hashcode are of use when the objects are used within collections.

Equals helps with collections that helps maintaining only unique objects ( like Set )

compare and compareTo helps with collections that helps maintaining objects in order ( TreeSet, TreeMap etc )

hascode helps with collections that facilitates hash searching ( like hashSet, hashMap etc )

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Q24. What will be the output of following ?

String str1 = new String("String1");
String str2 = new String("String1");
System.out.print(str1 == str2);
System.out.print(str1.equals(str2));
str1 = str2;
System.out.print(str1 == str2);

Core Java

a. falsetruetrue

b. truetruetrue

c. truetruefalse

d. falsetruefalse

Ans.a. falsetruetrue

Q25. What will be the output of following code ?

String str1 = "String1";
String str2 = "String1";
System.out.print(str1 == str2);
System.out.print(str1.equals(str2));
str1 = str2;
System.out.print(str1 == str2);

Core Java

a. falsetruetrue

b. falsefalsetrue

c. truetruetrue

d. falsefalsefalse

Ans.c. truetruetrue

Q26. Which of the following is false ?

Core Java

a. A Class cannot override both hashcode and equals method.

b. A class can override both hashcode and equals method.

c. A Class must override hashCode method if its overridding equal method.

d. A Class can override hashCode even if its not overridding equals method.

Ans.a. A Class cannot override both hashcode and equals method.

Q27. Which of the following is true for == operator ?

Core Java

a. For primitives, == checks if the variables on left and right have same data type

b. For primitives, == checks if the variables on left and right have same value

c. For Objects, == checks if the references on left and right have same data type

d. For Objects, == checks if the references on left and right have same value

Ans.b. For primitives, == checks if the variables on left and right have same value

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