Java – Page 32 – Program Creek

What do Java objects look like in memory during run-time?

July 10, 2017November 6, 2011 by ProgramCreek

As you may already know that once an object is created, it’s just a series of bytes in the heap. You may be curious about how exactly Java objects look like in the memory? 1. Fields Here is an example of an object layout for class “Base”(B). This class does not have any method. We … Read more

Java Access Level for Members: public, protected, private

May 3, 2014November 1, 2011 by ProgramCreek

Java access level contains two parts: 1) access level for classes and 2) access level for members. For class access level, the keyword can be public or no explicit modifier(package-private). For member access level, the keyword can be public, protected, package-private (no explicit modifier), or private. The following table summarizes the access level of different … Read more

When to use private constructors in Java?

December 28, 2013November 1, 2011 by ProgramCreek

If a method is private, it means that it can not be accessed from any class other than itself. This is the access control mechanism provided by Java. When it is used appropriately, it can produce security and functionality. Constructors, like regular methods, can also be declared as private. You may wonder why we need … Read more

How Java Compiler Generate Code for Overloaded and Overridden Methods?

February 25, 2013October 31, 2011 by ProgramCreek

Here is a simple Java example showing Polymorphism: overloading and overriding. Polymorphism means that functions assume different forms at different times. In case of compile time it is called function overloading. Overloading allows related methods to be accessed by use of a common name. It is sometimes called ad hoc polymorphism, as opposed to the … Read more

What is Instance Initializer in Java?

January 23, 2016October 29, 2011 by ProgramCreek

In this post, I will illustrate what are instance variable initializer, instance initializer, static initializer, and how the instance initializer works in Java. 1. Execution Order Look at the following class, do you know which one gets executed first? public class Foo { //instance variable initializer String s = "abc"; //constructor public Foo() … Read more

Sort a Double LinkedList in Java

September 27, 2013September 27, 2011 by ProgramCreek

This code is for showing how to sort a Double LinkedList in Java. You can use sort(List<T> list) method. It will sort the double list in ascending order. If you want to sort it in descending order, you can use sort(List<T> list, Comparator<? super T> c) . public static void main(String[] args) { LinkedList<Double> doubleList … Read more

A Puzzle from “A Brief History of the (Java) World and a Peek Forward” Presented by Neal Gafter

February 17, 2013September 18, 2011 by ProgramCreek

Here is a Java puzzle from Neal Gafter’s presentation. Which one is correct, a, b, c or d? The answer is d. There are 3 defects in this program: 1. Apparently, there is no “break” in the switch statement. 2. nextInt(2) will only return 0 or 1. 3. ‘P’, ‘G’, ‘M’ are Chars, not Strings. … Read more

Yet Another “Java Passes By Reference or By Value”?

December 26, 2013August 13, 2011 by ProgramCreek

This is a classic interview question which confuses novice Java developers. In this post I will use an example and some diagram to demonstrate that: Java is pass-by-value. 1. Some Definitions Pass by value: make a copy in memory of the actual parameter’s value that is passed in. Pass by reference: pass a copy of … Read more

Java Force Garbage Collection – Code Example

February 13, 2014July 17, 2011 by ProgramCreek

Garbage collection in java can not be enforced. But still sometimes, we call the System.gc( ) method explicitly. System.gc() method provides just a “hint” to the JVM that garbage collection should run. It is not guaranteed!

Java equals() and hashCode() Contract

June 19, 2017July 16, 2011 by ProgramCreek

The Java super class java.lang.Object defines two important methods: public boolean equals(Object obj) public int hashCode()public boolean equals(Object obj) public int hashCode() In this post, I will first show an example of a common mistake, and then explain how the equals() and hashCode() contract works. 1. A common mistake The common mistake is shown in … Read more

How to Build Your Own Java library?

December 29, 2013July 4, 2011 by ProgramCreek

Code reuse is one of the most important factors in software development. It is a VERY good idea to put frequently-used functions together and build a library for yourself. Whenever some method is used, just simply make a method invocation. For Java, it’s straightforward to manage such a library. Here a simple example in Eclipse. The library will contain only one “add” method for demo purpose.

Step 1: Create a “Java Project” named as “MyMath”, and a simple “add” method under “Simple” class.

Package structure is as follows:

Convert BST to Sorted Doubly Linked List (Java)

May 18, 2019May 18, 2011 by ProgramCreek

Let each node’s left child be the next smaller node and each node’s right child be the next bigger node. Node prev = null; Node head = null; public Node treeToDoublyList(Node root) { if (root == null) { return null; } helper(root); head.left = prev; prev.right = head; return head; } … Read more

Check a character in a String is number or not in Java

October 14, 2013April 11, 2011 by ProgramCreek

All you need is this simple function in java.lang package. Character.isDigit(c)Character.isDigit(c) Of course, if want to check a character in a string, need to get the char first by using the following. str.charAt(i)str.charAt(i)

A method to detect if string contains only uppercase letter in Java

October 6, 2013April 11, 2011 by ProgramCreek

My approach This method loop through each character in the string, and determine if each character is in upper case. Upper case letter value is from 97 to 122. public static boolean testAllUpperCase(String str){ for(int i=0; i<str.length(); i++){ char c = str.charAt(i); if(c >= 97 && c <= 122) { return false; } } //str.charAt(index) … Read more

Java method for spliting a camelcase string

August 9, 2015March 7, 2011 by ProgramCreek

This Java method accepts a camel case string, and returns a linked list of splitted strings. It has been designed to handle various kinds of different cases and fully tested and good to use. Handling camel case in Java involves a lot of styles, this methods can handle the following cases: Regular camel case names, … Read more