Java

Top 10 questions about Java Collections

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The following are the most popular questions of Java collections asked and discussed on Stackoverflow. Before you look at those questions, it’s a good idea to see the class hierarchy diagram. 1. When to use LinkedList over ArrayList? ArrayList is essentially an array. Its elements can be accessed directly by index. But if the array … Read more

Java Reflection Tutorial

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What is reflection? “Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine.” This concept is often mixed with introspection. The following are their definitions from Wiki: Introspection is the ability of a program to examine the type or properties … Read more

Top 10 Methods for Java Arrays

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The following are top 10 methods for Java Array. They are the most voted questions from stackoverflow. 0. Declare an array String[] aArray = new String[5]; String[] bArray = {"a","b","c", "d", "e"}; String[] cArray = new String[]{"a","b","c","d","e"};String[] aArray = new String[5]; String[] bArray = {"a","b","c", "d", "e"}; String[] cArray = new String[]{"a","b","c","d","e"}; 1. Print an … Read more

What Is Inner Interface in Java?

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What is Inner Interface in Java?

Inner interface is also called nested interface, which means declare an interface inside of another interface. For example, the Entry interface is declared in the Map interface.

public interface Map {
	interface Entry{
		int getKey();
	}
 
	void clear();
}

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Algorithm Problem Classification

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An algorithm problem contains 3 parts: input, output and solution/algorithm. The input can be an array, string, matrix, tree, linked list, graph, etc. The algorithm solution can be dynamic programming, binary search, BFS, DFS, or topological sort. The solution can also be a data structure, such as a stack, queue/dequeue, hash set, tree set, hash … Read more

Frequently Used Methods of Java HashMap

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HashMap is very useful when a counter is required.

HashMap<String, Integer> countMap = new HashMap<String, Integer>();
 
//.... a lot of a's like the following
if(countMap.keySet().contains(a)){
	countMap.put(a, countMap.get(a)+1);
}else{
	countMap.put(a, 1);
}

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Why String is immutable in Java?

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String is immutable in Java. An immutable class is simply a class whose instances cannot be modified. All information in an instance is initialized when the instance is created and the information can not be modified. There are many advantages of immutable classes. This article summarizes why String is designed to be immutable. This post illustrate the immutability concept in the perspective of memory, synchronization and data structures.

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How to Convert Array to ArrayList in Java?

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This article analyzes answers for a top-voted questions on Stack Overflow. The person who asked this question got a lot of reputation points, which could grant him permissions to do a lot of things on Stack Overflow. This does not make sense to me, but let’s take a look at the question first.

The question is “how to convert the following array to an ArrayList?”.

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What can we learn from Java HelloWorld?

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This is the program every Java programmer knows. It is simple, but a simple start can lead to deep understanding of more complex concepts. In this post I will explore what can be learned from this simple program. Please leave your comments if hello world means more to you.

HelloWorld.java

 
public class HelloWorld {
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("Hello World");
	}
}

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