SCJP : API Contents autoboxing & unboxing Mock Exam Practice Questions

Questions no -1 
What is the output for the below code ?

public class Test {
	

	public static void main(String[] args) {
		Boolean expr = true;
		if (expr) {
			System.out.println("true");
		} else {
			System.out.println("false");
		}
		
	}


}




options
A)true
B)Compile Error - can't use Boolean object in if().
C)false
D)Compile Properly but Runtime Exception.

Correct answer is : A

Explanations : In the if statement, condition can be Boolean object in jdk1.5 and jdk1.6.
In the previous version only boolean is allowed.

Questions no -2 
What is the output for the below code ?

public class Test {
	

	public static void main(String[] args) {
		List list = new ArrayList(); 
		list.add(0, 59);
		int total = list.get(0);
		System.out.println(total);

		
	}


}




options
A)59
B)Compile time error, because you have to do int total = ((Integer)(list.get(0))).intValue();
C)Compile time error, because can't add primitive type in List.
D)Compile Properly but Runtime Exception.

Correct answer is : A

Explanations :Manual conversion between primitive types (such as an int) and wrapper classes (such as Integer) is necessary when adding a primitive data type to a collection in jdk1.4 but The new autoboxing/unboxing feature eliminates this manual conversion in jdk 1.5 and jdk 1.6.

Questions no -3 
What is the output for the below code ?

public class Test {
	

	public static void main(String[] args) {
		Integer i = null;
		int j = i;
		System.out.println(j);
		
	}


}



options
A)0
B)Compile with error 
C)null
D)NullPointerException

Correct answer is : D

Explanations :An Integer expression can have a null value. If your program tries to autounbox null, it will throw a NullPointerException.