An Overview of JAVA From basic to slightly less basic Jonathan-Lee Jones

Overview Why JAVA? The VERY basics Data Types Arrays Functions & Methods and more advanced stuff Examples

Why JAVA? The programs that we are writing are very similar to their counterparts in several other languages, so our choice of language is not crucial. We use Java because it is widely available, embraces a full set of modern abstractions, and has a variety of automatic checks for mistakes in programs, so it is suitable for learning to program. There is no perfect language, and you certainly will be programming in other languages in the future.

The VERY Basics Programming in Java. We break the process of programming in Java into three steps: Create the program by typing it into a text editor and saving it to a file named, say, MyProgram.java. Compile it by typing "javac MyProgram.java" in the terminal window. Run (or execute) it by typing "java MyProgram" in the terminal window. The first step creates the program; the second translates it into a language more suitable for machine execution (and puts the result in a file named MyProgram.class); the third actually runs the program.

The VERY Basics public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, World"); }

The VERY Basics class UseArgument { public static void main(String[] args) { System.out.print("Hello, "); System.out.print(args[0]); System.out.print(", "); System.out.print(args[1]); System.out.print(", "); System.out.print(args[2]); System.out.println("!"); }

The VERY Basics class UseArgument { public static void main(String[] args) { if(args.length>0){ System.out.print("Hello, "); System.out.print(args[0]); System.out.print(", "); System.out.print(args[1]); System.out.print(", "); System.out.print(args[2]); System.out.println("!"); System.out.println } Else System.out.println(“Error!”); }

Data Types There are many built in types in JAVA. These built in types can be primitive types, or object types. There are more options available as to what the programmer can do with the more complicated objects than the simple built in primitive types. intcharlong IntegerStringdouble floatDoubleboolean

Arrays Arrays can be though of as a set of values. They can be of any data type (and can even be of user made objects, or other arrays). You represent an array by using the square brackets [] after the data type, and is created using the new command and giving it a size. Eg. int[] numbers = new int[10]; An array is used when you want to store a series of values. The most common array you have all used already! Public static void main(String[] args) This line, in all of your programs contains the array args. It is an array of type String, and is used to store all the command line arguments input by the user.

Arrays When you create a new array using the “new” command, you initialise all the values in it to 0. An array is a series of memory locations, and the variable name points the first position. In JAVA, you can then index each position in the array by using Array[i] where I is the position you want to read. In C++ this is trickier, as you need to know the bit-size of each element to move through memory. If you use “System.out.println(ArrayName);” then you will print the pointer to the memory address the array starts at. Array[0] Array[1] Array[2] Array[3] Array[4] Array

Arrays This code will create an array of numbers between 0 and n, where n is specified by the user. In JAVA arrays start at 0, so the first value would be array[0]. public class createArray { public static void main(final String[] args) { final int n = Integer.parseInt(args[0]); int[]numbers = new int[n] for(int i=0; i<n; ++i) numbers[i] = I; }

Functions & Methods It is often easier to write functions to do a certain task. For example, if you know you are going to require a certain type of calculation repeatedly. Functions are usually of the type static, and methods dynamic, but apart from this they are very similar. public class Max2 { public static int max(final int a, final int b) { if (a>b) return a; else return b; } public static void main(final String[] args) { final int a = Integer.parseInt(args[0]); final int b = Integer.parseInt(args[1]); System.out.println("The maximum of " + a + " and " + b + " is " + max(a,b) + "."); }

Functions & Methods Functions can also be overloaded. This means that you can have a different function (in this case max) with the same name, but taking different inputs. JAVA identifies the correct one to use at compile time. public class Max4 { public static int max(final int a, final int b) { if (a>b) return a; else return b; } public static int max(final int a, final int b, final int c) { return max(max(a,b),c); } public static int max(final int a, final int b, final int c, final int d) { return max(max(a,b,c),d); } public static void main(final String[] args) { final int a = Integer.parseInt(args[0]); final int b = Integer.parseInt(args[1]); final int c = Integer.parseInt(args[2]); final int d = Integer.parseInt(args[3]); StdOut.printf("The maximum of %d, %d, %d, %d is %d.\n", a,b,c,d,max(a,b,c,d)); }

Functions & Methods The term methods usually refers to dynamically called functions. These are not determined at compile time, but are called implicitly on an object. For example this.minVal() would run the minVal method on this object. Methods are often used when the user creates objects of their own. A good example of this is a block object

Functions & Methods class Block{ private double height; private double width; private double length; public Block(double height, double width, double length) { this.height=height; this.width=width; this.length=length; } public double getTopArea() //legth x width { return length*width;} public double getFrontArea() //length x height {return length*height;} public double getSideArea() //width x height {return width*height;} public double getVolume() //area x height {return length*height*width;} }

Functions & Methods class BlockTest{ public static void main(String[] args){ if(args.length<3) { System.err.println(“Not enough arguments”); return; } else { double L = Double.parseDouble(args[0]); double W = Double.parseDouble(args[0]); double H = Double.parseDouble(args[0]); Block testB = new Block(L,W,H); System.out.println(“Volume = “ + testB.getVolume()); }

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