Code Blocks

I. Code Block

In java, the code inside a pair of {} called code block.

  • Local code block:
    • Inside a method
    • To limit the lifecycle of variable
    • Release varaible to improve the performance of memory usage.
  • Construction code block:
    • In the position of field of a class
    • Will be executed before the constructor has been executed
    • To put common codes of all constructors together
  • Static code block
    • In the position of field of a class, with static
    • To initialize the class
  • Synchronized code block
    • (to be continued)
class Code {
	{
	// Construction Code Block
		int x = 10;
		System.out.println(x);
	}

	public Code(){}

	{
	// Construction Code Block
		int y = 100;
		System.out.println(x);
	}

	static {
	// Static Code Block
		int z = 200;
		System.out.println(x);
	}

	public static void main(String[] args){
		{
			// Local Code Block
			int y = 20;
			System.out.println(y);
		}
	}
}

II. Question

The execution squence of local code block, construction code block, and static code block?

Answer: static code block –> construction code block –> constructor

static code block construction code block –> constructor
Executed only once when the class has been loaded Executed when the constructor has been called

Code:

package org.lovain.codeblock;

/**
 * Code Block Demo: Student Class
 * @author PENG Zhengshuai
 * @lovian.org
 *
 */
public class Student {
	private String name;

	static {
		System.out.println("Student staitic code block");
	}

	{
		System.out.println("Student construction code block");
	}

	public Student(String name) {
		System.out.println("Student" + name + " constructor");
	}
}


package org.lovain.codeblock;

/**
 * Code Block Demo: Demo Class
 * @author PENG Zhengshuai
 * @lovian.org
 *
 */
public class StudentDemo {
	static {
		System.out.println("Demo static code block");
	}

	public static void main(String[] args) {
		System.out.println("Demo main method");

		Student s1 = new Student("James");
		Student s2 = new Student("Xiaoming");
	}
}

Result:

Demo static code block
Demo main method
Student staitic code block
Student construction code block
StudentJames constructor
Student construction code block
StudentXiaoming constructor
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