Compile Error Vs runtime Error

[Harikrishna Gorrepati]

Hi,

Why I am getting one error in loop during compile time and similar error during Runtime in the other loop?

public class Question19
{
	Boolean b[] = new Boolean[5];
	
	public void method()
	{
		if(null == true)	//Why compile time error.
			System.out.println("Compie Error");
		
		if(b[1] == true)	//Why runtime Error. Null Pointer Exception
			System.out.println("Runtime Error: Null Pointer Exception");
	}
	
	public static void main(String[] args)
	{
		new Question19().method();
	}
}

[Wouter Oet]

First of all please QuoteYourSources.

The compile time error is because you're trying to compare a null (reference to nothing) to a primitive boolean. That can't be done. It's like comparing apples to your left shoe. They are completely different.

The NullPointerException is caused by auto-unboxing of the second value of the Boolean array. Note that it is a wrapper Boolean not a primitive thus it's default value is null.

[Arjun Srivastava]

Wouter Oet wrote:The compile time error is because you're trying to compare a null (reference to nothing) to a primitive boolean. That can't be done. It's like comparing apples to your left shoe. They are completely different.
The NullPointerException is caused by auto-unboxing of the second value of the Boolean array. Note that it is a wrapper Boolean not a primitive thus it's default value is null.

public class Question19
{
	Boolean b[] = new Boolean[5];
	
	public void method()
	{
		if(null ==new Object())	//NO compile time error.
			System.out.println("Compie Error");
		
		if(b[1] == new Object())	//NO runtime Error. Null Pointer Exception
			System.out.println("Runtime Error: Null Pointer Exception");
	}
	
	public static void main(String[] args)
	{
		new Question19().method();
	}
}

this time it works fine*
as null is comparable to object,also wrapper classes,is it so?

[Trivikram Kamat]

Arjun Srivastava wrote:
this time it works fine*
as null is comparable to object,also wrapper classes,is it so?

Yeah, it must be the correct reason.
An Object can be given a null reference. So, it can be compared to a null.
And all classes are derived from Object class in Java (except Object class of course). So, instances of Wrapper classes can be compared to that of Object class.