Static blocks in Java

Introduction

Static blocks run code once when the class loads. They help set up things before you use the class.

To initialize static variables with complex logic.

To run setup code once before any object is created.

To load configuration or resources when the program starts.

To print a message or log when the class is first used.

Syntax

Java

static {
    // code to run once when class loads
}

Static blocks run only once, when the class is loaded by the Java Virtual Machine.

You can have multiple static blocks; they run in the order they appear.

Examples

This static block prints a message when the class loads.

Java

class Example {
    static {
        System.out.println("Static block runs");
    }
}

This static block sets a static variable with a calculation.

Java

class Example {
    static int value;
    static {
        value = 10 * 2;
    }
}

Multiple static blocks run in order when the class loads.

Java

class Example {
    static {
        System.out.println("First block");
    }
    static {
        System.out.println("Second block");
    }
}

Sample Program

This program shows a static block that runs once before main. It sets the static variable number. The main method then prints the value.

Java

public class StaticBlockDemo {
    static int number;
    static {
        System.out.println("Static block is running");
        number = 5 * 5;
    }
    public static void main(String[] args) {
        System.out.println("Number is: " + number);
    }
}

Important Notes

Static blocks cannot access instance variables directly because they run before any object exists.

If you have multiple static blocks, they run top to bottom in the class.

Static blocks are useful for one-time setup related to the class itself.

Summary

Static blocks run once when the class loads.

They are used to initialize static variables or run setup code.

Multiple static blocks run in the order they appear.

Practice

1. What is the main purpose of a static block in Java?

easy

A. To handle exceptions during runtime

B. To create instance variables for each object

C. To initialize static variables or run code once when the class loads

D. To define methods that can be overridden

Solution

1. Step 1: Understand static block behavior

   A static block runs once when the class is loaded, before any object is created.

2. Step 2: Identify its purpose

   It is mainly used to initialize static variables or run setup code that should execute once.

3. Final Answer:

   To initialize static variables or run code once when the class loads -> Option C

4. Quick Check:

   Static block purpose = initialize static variables [OK]

Hint: Static blocks run once at class load for static setup [OK]

Common Mistakes:

• Thinking static blocks run for each object
• Confusing static blocks with constructors
• Assuming static blocks handle exceptions only

2. Which of the following is the correct syntax for a static block in Java?

easy

A. static() { System.out.println("Hello"); }

B. static { System.out.println("Hello"); }

C. static void { System.out.println("Hello"); }

D. static class { System.out.println("Hello"); }

Solution

1. Step 1: Recall static block syntax

   A static block is declared with the keyword static followed by braces containing code, without parentheses or return types.

2. Step 2: Check options

   static { System.out.println("Hello"); } matches the correct syntax: static { ... }. Others incorrectly add parentheses, return types, or keywords.

3. Final Answer:

   static { System.out.println("Hello"); } -> Option B

4. Quick Check:

   Static block syntax = static { ... } [OK]

Hint: Static blocks have no parentheses or return type [OK]

Common Mistakes:

• Adding parentheses after static keyword
• Using void or other return types
• Trying to declare static block as a method

3. What will be the output of the following Java code?

class Test {
    static int x;
    static {
        x = 5;
        System.out.print("A");
    }
    static {
        x = 10;
        System.out.print("B");
    }
    public static void main(String[] args) {
        System.out.print(x);
    }
}

medium

A. AB10

B. BA10

C. AB5

D. BA5

Solution

1. Step 1: Understand static block execution order

   Static blocks run in the order they appear when the class loads. First block prints "A" and sets x=5, second prints "B" and sets x=10.

2. Step 2: Trace main method output

   Main prints the value of x after static blocks run, which is 10 (last assignment). So output is "AB10".

3. Final Answer:

   AB10 -> Option A

4. Quick Check:

   Static blocks run in order, last x=10 [OK]

Hint: Static blocks run top to bottom before main [OK]

Common Mistakes:

• Assuming static blocks run after main
• Confusing order of static blocks
• Expecting x to be 5 instead of 10

4. Identify the error in the following code snippet:

class Example {
    static {
        System.out.println("Start");
    }
    static void block() {
        System.out.println("Inside method");
    }
}

medium

A. No error, code is valid

B. Static block must have a method name

C. Static block cannot contain print statements

D. Static block cannot be followed by a static method

Solution

1. Step 1: Check static block rules

   Static blocks can contain code like print statements and do not require a method name.

2. Step 2: Verify static method declaration

   Static methods can appear after static blocks without any issue.

3. Final Answer:

   No error, code is valid -> Option A

4. Quick Check:

   Static blocks and methods can coexist [OK]

Hint: Static blocks can have code; methods are separate [OK]

Common Mistakes:

• Thinking static blocks need method names
• Assuming print statements are invalid in static blocks
• Believing static blocks and methods conflict

5. Consider this Java class:

class Config {
    static int value;
    static {
        value = 100;
        System.out.print("Init1-");
    }
    static {
        value += 50;
        System.out.print("Init2-");
    }
    static {
        if (value > 100) {
            value = 200;
            System.out.print("Init3-");
        }
    }
    public static void main(String[] args) {
        System.out.print(value);
    }
}

What is the output when this program runs?

hard

A. Init2-Init3-200

B. Init1-Init2-150

C. Init1-Init3-200

D. Init1-Init2-Init3-200

Solution

1. Step 1: Trace static blocks in order

   First block sets value=100 and prints "Init1-". Second adds 50 (value=150) and prints "Init2-". Third checks if value>100 (true), sets value=200 and prints "Init3-".

2. Step 2: Check main output

   Main prints the final value, which is 200 after all static blocks run.

3. Final Answer:

   Init1-Init2-Init3-200 -> Option D

4. Quick Check:

   Static blocks run sequentially, final value=200 [OK]

Hint: Static blocks run top to bottom, last changes persist [OK]

Common Mistakes:

• Ignoring the condition in third static block
• Assuming static blocks run in random order
• Forgetting value updates in each block