Java Tutorials - Herong's Tutorial Examples - v8.22, by Herong Yang
main() Method - Java Execution Entry Point
This section describes the Java program entry point, the main() method of the starting class. Command line arguments are passed as an string array parameter to the main() method.
As mentioned in the previous section, a Java application program must have a main class with a special method call main() as the execution entry point. The main() method must be defined as:
public static void main(String[] a) { // statement block }
Note that the main() method must be declared as "public static void".
The parameter of the main() method is an array of "String", which contains the additional arguments after the class name argument in the "java" command line. Let's use the following Java program to illustrate how those arguments are passed into the main() method:
/* CommandLine.java * Copyright (c) HerongYang.com. All Rights Reserved. */ class CommandLine { public static void main(String[] a) { System.out.println("Number of arguments = "+a.length); for (int i=0; i<a.length; i++) { System.out.println(" a["+i+"] = "+a[i]); } } }
After compiling the class, execute it with the following command line:
herong> javac CommandLine.java herong> java -classpath . CommandLine bread milk banana apple Number of arguments = 4 a[0] = bread a[1] = milk a[2] = banana a[3] = apple
4 arguments are specified in the command line after the class name. The JVM put them in an array of 4 elements and pass the array as a parameter to the main() method.
Execute it again with a slightly different command line:
herong> java -classpath . CommandLine "bread milk" "banana apple" Number of arguments = 2 a[0] = bread milk a[1] = banana apple
This time, double quotes are used to make two space delimited words as a single argument.
Table of Contents
►Execution Process, Entry Point, Input and Output
Creating, Compiling and Executing Java Programs
►main() Method - Java Execution Entry Point
Java Execution Console - "in", "out" and "err" Data Streams
Primitive Data Types and Literals
Bits, Bytes, Bitwise and Shift Operations
Managing Bit Strings in Byte Arrays
Reference Data Types and Variables
StringBuffer - The String Buffer Class
System Properties and Runtime Object Methods
Generic Classes and Parameterized Types
Generic Methods and Type Inference
Lambda Expressions and Method References
Java Modules - Java Package Aggregation
Execution Threads and Multi-Threading Java Programs
ThreadGroup Class and "system" ThreadGroup Tree
Synchronization Technique and Synchronized Code Blocks
Deadlock Condition Example Programs
Garbage Collection and the gc() Method
Assert Statements and -ea" Option