All algorithms in Java
Breadth-First Search in Java
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The Breadth-first search algorithm is an algorithm used to solve the shortest path problem in a graph without edge weights (i.e. a graph where all nodes are the same “distance” from each other, and they are either connected or not). This means that given a number of nodes and the edges between them, the Breadth-first search algorithm is finds the shortest path from the specified start node to all other nodes. Nodes are sometimes referred to as vertices (plural of vertex) - here, we’ll call them nodes.
See CodeA Star in Java
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The A star (A*) algorithm is an algorithm used to solve the shortest path problem in a graph. This means that given a number of nodes and the edges between them as well as the “length” of the edges (referred to as “weight”) and a heuristic (more on that later), the A* algorithm finds the shortest path from the specified start node to all other nodes. Nodes are sometimes referred to as vertices (plural of vertex) - here, we’ll call them nodes.
See CodeDFS in Java
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The Depth-First Search (also DFS) algorithm is an algorithm used to find a node in a tree. This means that given a tree data structure, the algorithm will return the first node in this tree that matches the specified condition (i.e. being equal to a value). Nodes are sometimes referred to as vertices (plural of vertex) - here, we’ll call them nodes. The edges have to be unweighted. This algorithm can also work with unweighted graphs if a mechanism to keep track of already visited nodes is added.
See CodeDijkstra in Java
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The Dijkstra algorithm is an algorithm used to solve the shortest path problem in a graph. This means that given a number of nodes and the edges between them as well as the “length” of the edges (referred to as “weight”), the Dijkstra algorithm is finds the shortest path from the specified start node to all other nodes. Nodes are sometimes referred to as vertices (plural of vertex) - here, we’ll call them nodes.
See CodeGreatest Common Divisor in Java
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The greatest common divisor of two numbers (in this case a and b) is the biggest number which both numbers can be divided by without a rest. This greatest common divisor algorithm, called the euclidean algorithm, determines this number. The greatest common divisor is also often abbreviated as gcd.
See CodeIterative Deepening A Star in Java
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The Iterative Deepening A Star (IDA*) algorithm is an algorithm used to solve the shortest path problem in a tree, but can be modified to handle graphs (i.e. cycles). It builds on Iterative Deepening Depth-First Search (ID-DFS) by adding an heuristic to explore only relevant nodes.
See CodeIterative Deepening DFS in Java
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The Iterative Deepening Depth-First Search (also ID-DFS) algorithm is an algorithm used to find a node in a tree. This means that given a tree data structure, the algorithm will return the first node in this tree that matches the specified condition. Nodes are sometimes referred to as vertices (plural of vertex) - here, we’ll call them nodes. The edges have to be unweighted. This algorithm can also work with unweighted graphs if mechanism to keep track of already visited nodes is added.
See CodePoint in Polygon in Java
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The Point in Polygon (PIP) problem is the problem of determining whether a point is any arbitrary polygon. This might sound trivial for a simple polygon like a square or a triangle, but gets more complex with more complex polygons like the one in the example below. In this post, the even-odd algorithm, also called crossing number algorithm or Jordan’s algorithm (since it can be proven using the Jordan curve theorem), will be introduced.
See CodeAbout the programming language:
Java
Java™ is a compiled language used for many purposes, ranging from embedded systems, UI-applications to web servers.
Getting to “Hello World” in Java
The most important things first - here’s how you can run your first line of code in Java.
- Download and install the latest version of Java from java.com. You can also download an earlier version if your use case requires it.
-
Open a terminal, make sure the
javac
andjavac
commands are working, and that the command your’re going to be using is referring to the version you just installed by runningjava -version
. If you’re getting a “command not found” error (or similar), try restarting your command line, and, if that doesn’t help, your computer. If the issue persists, here are some helpful StackOverflow questions for each platform: -
As soon as that’s working, copy the following snippet into a file named HelloWorld.java:
class HelloWorld { public static void main(String[] args) { // Paste any following code snippets here. System.out.println("Hello World"); } }
- Change directory by typing
cd path/to/HelloWorld
, then runjavac HelloWorld.java
to compile the file (which creates the bytecode), then runjava HelloWorld
(without the .java ending). This should print “Hello World” to your Terminal.
That’s it! Notice that the entry barrier is a little higher with Java than it is with e.g. Python - but Java is much faster and, in my experience, tends to have fewer bugs in large projects due to strong typing and other factors.
Fundamentals in Java
To understand algorithms and technologies implemented in Java, one first needs to understand what basic programming concepts look like in this particular language. Each of the following snippets should be surrounded by the boilerplate code of the hello world example and should be compiled and run using the commands mentioned above.
Variables and Arithmetic
Variables in Java are statically typed, meaning the content of a variable needs to be specified when writing the code.
The datatype for whole numbers, for example is int
.
Numbers with decimal places are typed float
or double
depending on the required precision.
The type for text ist String
.
int number = 5;
double decimalNumber = 3.25;
double result = number * decimalNumber;
String callout = "The number is ";
// In this instance, the values are concatenated rather than added because one of them is a String.
System.out.println(callout + result);
Arrays
Arrays in Java are real arrays (as opposed to e.g. Python where they’re implemented as lists). The implications of that are that the size needs to be set when they are created and cannot be changed, but also that they are more efficient in Java than they are in Python.
int[] integers = new int[5];
integers[3] = 12; // Assigning values to positions in the array
// integers[4] is 0, integers[6] would give IndexOutOfBoundsException
String[] strings = {"Hello", "World"}; // Array initialization with initial values
System.out.println(strings[0] + integers[3]); // Prints "Hello12"
Conditions
Just like most programming languages, Java can do if-else
statements. Additionally, Java can also do switch-case
statements.
int value = 5;
if(value == 5){
System.out.println("Value is 5");
} else if(value < 5){
System.out.println("Value is less than 5");
} else {
System.out.println("Value is something else");
}
switch (value){
case 1:
System.out.println("Value is 1");
break; // Don't go further down the cases
case 2:
System.out.println("Value is 2");
break; // Don't go further down the cases
case 3:
System.out.println("Value is 3");
break; // Don't go further down the cases
case 4:
System.out.println("Value is 4");
break; // Don't go further down the cases
case 5:
System.out.println("Value is 5");
break; // Don't go further down the cases
default:
System.out.println("Value is something else");
}
The above Java code will print “Value is 5” twice.
Loops
Java supports for
, while
as well as do while
loops. break
and continue
statements are also supported.
The below example illustrates the differences:
int value = 2;
for (int i = 0; i < value; i++) {
System.out.println(i);
}
while (value > 0) {
System.out.println(value);
value--;
}
do {
System.out.println(value);
value--;
} while (value > 0);
This will print the following to the terminal:
0
1
2
1
0
Note the last 0
: it is printed because in the do-while
-loop, compared to the while
-loop. the code block is executed at least once before the condition is checked.
Functions
Functions in Java can be part of a class, or of an object of a class. For more information on object oriented programming I recommend the w3schools course. Here is a minimal example of a function as part of a class (also called a static function):
class HelloWorld {
public static void main(String[] args) {
System.out.println(addNumbers(3, 4));
}
public static int addNumbers(int numberOne, int numberTwo) {
return numberOne + numberTwo;
}
}
And here’s an example of calling a function of an object of a class:
class HelloWorld {
public static void main(String[] args) {
System.out.println(new HelloWorld().addNumbers(3, 4));
}
public int addNumbers(int numberOne, int numberTwo) {
return numberOne + numberTwo;
}
}
Note how the first example uses the static
keyword, and the second example needs to instantiate on object of the class before in can call the function of that object.
These are some of the differences in class methods and object functions.
Syntax
Java requires the use of curly brackets ({}
) to surround code blocks in conditions, loops, functions etc.;
It also requires semicolons at then end of statements.
While this can lead to some annoying syntax errors, it also means the use of whitespace for preferred formatting (e.g. indentation of code pieces) does not affect the code.
Advanced Knowledge of Java
Java was first released in 1995 and is multi-paradigm, meaning while it is primarily object-oriented, it also has functional and reflective elements. It’s statically typed, but offers some amount of dynamic typing in recent versions. For more information, Java has a great Wikipedia) article.