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Difficulty level: medium

Predict the output of the following Java program:

The problem

Problem a

class Gfg
{
    // constructor
    Gfg()
    {
        System.out.println("juejin");
    }
      
    static Gfg a = new Gfg(); //line 8
  
    public static void main(String args[])
    {
        Gfg b; //line 12
        b = newGfg(); }}Copy the code

Click here to skip to the answer

Question 2

class Gfg
{
    static int num;
    static String mystr;
  
    // constructor
    Gfg()
    {
        num = 100;
        mystr = "Constructor";
    }
  
    // First Static block
    static
    {
        System.out.println("Static Block 1");
        num = 68;
        mystr = "Block1";
    }
  
    // Second static block
    static
    {
        System.out.println("Static Block 2");
        num = 98;
        mystr = "Block2";
    }
  
    public static void main(String args[])
    {
        Gfg a = new Gfg();
        System.out.println("Value of num = " + a.num);
        System.out.println("Value of mystr = "+ a.mystr); }}Copy the code

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Question 3

class superClass
{
	final public int calc(int a, int b)
	{
		return 0; }}class subClass extends superClass
{
	public int calc(int a, int b)
	{
		return 1; }}public class Gfg
{
	public static void main(String args[])
	{
		subClass get = new subClass();
		System.out.println("x = " + get.calc(0.1)); }}Copy the code

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Problem four

public class Gfg
{
	public static void main(String[] args)
	{
		Integer a = 128, b = 128;
		System.out.println(a == b);

		Integer c = 100, d = 100; System.out.println(c == d); }}Copy the code

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Put a picture of a cute girl to relieve eye fatigue, the second half of the article is the output and analysis of the program

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Output and parsing

Answer to question one

Output:

juejin
juejin
Copy the code

Explanation:

We know that static variables are called when the class loads, and static variables are only called once. Line 13 now causes the creation of the object, which in turn calls the constructor, printing “juejin” a second time. If static variables are not used in line 8, the object is called recursively indefinitely, resulting in a StackOverFlow error. See this example run.

Answer to question two

Output:

Static Block 1
Static Block 2
Value of num = 100
Value of mystr = Constructor
Copy the code

Description:

Static blocks are executed when the class is loaded into memory. A class can have multiple static blocks that are executed in the same order in which they were written to the program.

Note: Static methods can access class variables without using the object of the class. Since the constructor is called when a new instance is created, the static block is called first, followed by the constructor. If we run the same program without using an object, the constructor is not called.

Answer to Question three

Output:

Compilation fails
Copy the code

Explanation:

The calc() method in the superClass class is final and therefore cannot be overridden.

Answer to Question 4

Output:

false
true
Copy the code

Description:

Note: In the source code for the Integer object, we find a method ‘valueOf’. In this method, we can see that the Integer object ranges from integerCache.low (-128) to integerCache.high (127). Therefore, numbers above 127 do not give the expected output. The scope of IntegerCache can be seen in the IntegerCache class source code.

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