1. Sridhar Narayan narayans@uncw.edu
Java Basics
Sridhar Narayan

2. Java programs A Java program is a collection of classes
Each class is a generic description (template) of an object being modeled
Each class describes
Properties of the object being modeled, i.e. structure of the object
Ways in which one can interact with the object, i.e. behavior of the object. These are called methods.
The main method in a class is where execution begins when that class is run

3. Java methods Each method is a collection of statements
Each statement is terminated by a semi-colon
Java is case sensitive
Each variable must be declared before use
int, float, double, char, boolean are primitive types
Every Java class is a non-primitive type
Java employs block scoping
Each pair of {} defines a block
Variables declared within a block are visible only within that block

4. Java comments // single line comment /* multi-line comment */
/** used for Java docs, similar to Python docstring comments **/

5. Java types
Unlike Python, Java does not infer types. Types are explicitly declared
int x = 10;
boolean p = false;
Java is strongly typed.
float x = 2.5; //error, since 2.5 is a double
float x = 2.5f; // ok
Statically type checked
All type errors are caught during compilation

6. Compiled and then interpreted
The java compiler first compiles Java source code (.java files) into Java byte code (.class files)
The byte code is then interpreted by the Java VM (virtual machine)
In theory, once compiled, Java code can be run on any platform for which a JVM exists.

7. Iterative constructs for(int i=0; i < 10; i++) { //do something }
while (condition is true) { //do something }
do { //something } while (condition is true);

8. Selection if- else if - else Use parenthesis for conditions:
if (a > b) { //do something } else if (c > d) { //do something else } else { //do a third thing }
&& (and), || (or)
if (a > b && c < d || a == b) { //do something }

9. Arrays int x; //declare x as an integer. x is a primitive
int [] data; // declare data as a reference to an array of integers. No array exists at this point
data = new int[100]; //instantiate and assign an array of 100 integers to data
Declaration and instantiation can be combined
int [] data = new int[100];
Array index values start with 0. For example above, valid references are data[0]…data[99]