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Core Java Interview Questions and Answers

Ques 161. How can you force garbage collection?

You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

The following code of program will help you in forcing the garbage collection. First of all we have created an object for the garbage collector to perform some operation. Then we have used the System.gc(); method to force the garbage collection on that object. Then we have used the System.currentTimeMillis(); method to show the time take by the garbage collector.


import java.util.Vector;

public class GarbageCollector{
	public static void main(String[] args) {
		int SIZE = 200;
		StringBuffer s;
		for (int i = 0; i < SIZE; i++) {
		}
		System.out.println("Garbage Collection started explicitly.");
		long time = System.currentTimeMillis();
		System.gc();
		System.out.println("It took " +(System.currentTimeMillis()-time) + " ms");
	}
}

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Ques 162. What comes to mind when you hear about a young generation in Java?

Garbage collection.


In the J2SE platform version 1.4.1 two new garbage collectors were introduced to make a total of four garbage collectors from which to choose.
Beginning with the J2SE platform, version 1.2, the virtual machine incorporated a number of different garbage collection algorithms that are combined using generational collection. While naive garbage collection examines every live object in the heap, generational collection exploits several empirically observed properties of most applications to avoid extra work.

The default collector in HotSpot has two generations: the young generation and the tenured generation. Most allocations are done in the young generation. The young generation is optimized for objects that have a short lifetime relative to the interval between collections. Objects that survive several collections in the young generation are moved to the tenured generation. The young generation is typically smaller and is collected more often. The tenured generation is typically larger and collected less often.

The young generation collector is a copying collector. The young generation is divided into 3 spaces: eden-space, to-space, and from-space. Allocations are done from eden-space and from-space. When those are full a young generation is collection is done. The expectation is that most of the objects are garbage and any surviving objects can be copied to to-space. If there are more surviving objects than can fit into to-space, the remaining objects are copied into the tenured generation. There is an option to collect the young generation in parallel.

The tenured generation is collected with a mark-sweep-compact collection. There is an option to collect the tenured generation concurrently.

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Ques 163. How does exception handling work in Java?

1.It separates the working/functional code from the error-handling code by way of try-catch clauses.
2.It allows a clean path for error propagation. If the called method encounters a situation it can't manage, it can throw an exception and let the calling method deal with it.
3.By enlisting the compiler to ensure that "exceptional" situations are anticipated and accounted for, it enforces powerful coding.
4.Exceptions are of two types: Compiler-enforced exceptions, or checked exceptions. Runtime exceptions, or unchecked exceptions. Compiler-enforced (checked) exceptions are instances of the Exception class or one of its subclasses '?? excluding the RuntimeException branch. The compiler expects all checked exceptions to be appropriately handled. Checked exceptions must be declared in the throws clause of the method throwing them '?? assuming, of course, they're not being caught within that same method. The calling method must take care of these exceptions by either catching or declaring them in its throws clause. Thus, making an exception checked forces the us to pay heed to the possibility of it being thrown. An example of a checked exception is java.io.IOException. As the name suggests, it throws whenever an input/output operation is abnormally terminated.

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Ques 164. Does Java have destructors?

Garbage collector does the job working in the background
Java does not have destructors; but it has finalizers that does a similar job.
the syntax is
public void finalize(){
}
if an object has a finalizer, the method is invoked before the system garbage collects the object

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Ques 165. What is the catch or declare rule for method declarations?

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

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