Generics Restrictions - Java Questions & Answers

This set of Java Multiple Choice Questions & Answers (MCQs) focuses on “Restrictions on Generics”.

1. Which of these types cannot be used to initiate a generic type?
a) Integer class
b) Float class
c) Primitive Types
d) Collections
View Answer

Answer: c
Explanation: None.

2. Which of these instance cannot be created?
a) Integer instance
b) Generic class instance
c) Generic type instance
d) Collection instances
View Answer

Answer: c
Explanation: It is not possible to create generic type instances. Example – “E obj = new E()” will give a compilation error.

3. Which of these data type cannot be type parameterized?
a) Array
b) List
c) Map
d) Set
View Answer

Answer: a
Explanation: None.

4. What will be the output of the following Java program?

```
    public class BoxDemo
```
```
    {
```

        public static <U> void addBox(U u,

           java.util.List<Box<U>> boxes)

```
          {
```
```
            Box<U> box = new Box<>();
```
```
            box.set(u);
```
```
            boxes.add(box);
```
```
          }
```

        public static <U> void outputBoxes(java.util.List<Box<U>> boxes)

```
        {
```
```
            int counter = 0;
```
```
            for (Box<U> box: boxes)
```
```
            {
```

                U boxContents = box.get();

                System.out.println("Box #" + counter + " contains [" + boxContents.toString() + "]");

```
                counter++;
```
```
            }
```
```
        }
```

        public static void main(String[] args)

```
        {
```

            java.util.ArrayList<Box<Integer>> listOfIntegerBoxes = new java.util.ArrayList<>();

            BoxDemo.<Integer>addBox(Integer.valueOf(10), listOfIntegerBoxes);

            BoxDemo.outputBoxes(listOfIntegerBoxes);

```
        }
```
```
    }
```

a) 10
b) Box #0 [10]
c) Box contains [10]
d) Box #0 contains [10]
View Answer

Answer: d
Explanation: None.
Output:

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$ javac Output.java
$ java Output
Box #0 contains [10]

5. What will be the output of the following Java program?

```
    import java.util.*;
```
```
    public class genericstack <E>
```
```
    {
```

        Stack <E> stk = new Stack <E>();

```
	public void push(E obj) 
```
```
        {
```
```
            stk.push(obj);
```
```
	}
```
```
	public E pop() 
```
```
        {
```
```
            E obj = stk.pop();
```
```
	    return obj;
```
```
	}
```
```
    }
```
```
    class Output
```
```
    {
```

        public static void main(String args[])

```
        {
```

            genericstack <String> gs = new genericstack<String>();

```
            gs.push("Hello");
```

            System.out.print(gs.pop() + " ");

            genericstack <Integer> gs = new genericstack<Integer>();

```
            gs.push(36);
```

            System.out.println(gs.pop());

```
        }
```
```
    }
```

a) Error
b) Hello
c) 36
d) Hello 36
View Answer

Answer: d
Explanation: None.
Output:

$ javac Output.java
$ java Output
Hello 36

6. What will be the output of the following Java program?

```
    import java.util.*;
```
```
    class Output
```
```
    {
```

        public static double sumOfList(List<? extends Number> list)

```
        {
```
```
            double s = 0.0;
```
```
            for (Number n : list)
```
```
                s += n.doubleValue();
```
```
            return s;
```
```
        }
```

        public static void main(String args[])

```
        {
```

           List<Double> ld = Arrays.asList(1.2, 2.3, 3.5);

           System.out.println(sumOfList(ld));

```
        }
```
```
    }
```

a) 5.0
b) 7.0
c) 8.0
d) 6.0
View Answer

Answer: b
Explanation: None.
Output:

$ javac Output.java
$ java Output
7.0

7. What will be the output of the following Java program?

```
    import java.util.*;
```
```
    class Output
```
```
    {
```

        public static void addNumbers(List<? super Integer> list)

```
        {
```

            for (int i = 1; i <= 10; i++)

```
            {
```
```
                list.add(i);
```
```
            }
```
```
        }
```

        public static void main(String args[])

```
        {
```

           List<Double> ld = Arrays.asList();

```
           addnumbers(10.4);
```

           System.out.println("getList(2)");

```
        }
```
```
    }
```

a) 1
b) 2
c) 3
d) 6
View Answer

Answer: a
Explanation: None.
Output:

$ javac Output.java
$ java Output
1

8. What will be the output of the following Java program?

```
    import java.util.*;
```
```
    public class genericstack <E>
```
```
    {
```

        Stack <E> stk = new Stack <E>();

```
	public void push(E obj)
```
```
        {
```
```
            stk.push(obj);
```
```
	}
```
```
	public E pop()
```
```
        {
```
```
            E obj = stk.pop();
```
```
	    return obj;
```
```
	}
```
```
    }
```
```
    class Output
```
```
    {
```

        public static void main(String args[])

```
        {
```

            genericstack <Integer> gs = new genericstack<Integer>();

```
            gs.push(36);
```

            System.out.println(gs.pop());

```
        }
```
```
    }
```

a) H
b) Hello
c) Runtime Error
d) Compilation Error
View Answer

Answer: d
Explanation: generic stack object gs is defined to contain a string parameter but we are sending an integer parameter, which results in compilation error.
Output:

$ javac Output.java

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