Static blocks in Java - Step-by-Step Execution

Concept Flow - Static blocks

Class Load Start

↓

Static Variables Initialized

↓

Execute Static Block

↓

Class Ready for Use

↓

Main Method or Object Creation

When the class loads, the static block runs once to initialize static variables before any object is created or main runs.

Execution Sample

Java

class Test {
  static int x;
  static {
    x = 5;
    System.out.println("Static block runs");
  }
  public static void main(String[] args) {
    System.out.println(x);
  }
}

This code runs the static block once when the class loads, sets x to 5, then prints x in main.

Execution Table

Step	Action	Static Variable x	Output
1	Class loads, static variables initialized	0
2	Static block runs, sets x to 5	5	Static block runs
3	Main method starts, print x	5	5
4	Program ends	5

💡 Program ends after main method finishes

Variable Tracker

Variable	Start	After Static Block	After Main
x	0 (default)	5	5

Key Moments - 3 Insights

Why does the static block run before main?

The static block runs when the class loads, before main starts, as shown in execution_table step 2.

Does the static block run again if we create objects?

No, static blocks run only once when the class loads, not on object creation.

What is the initial value of static int x before the static block?

It is 0 by default, as shown in variable_tracker start value.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of x after the static block runs?

A 0

B Undefined

C 5

D 10

💡 Hint

Check execution_table row 2 under 'Static Variable x'

At which step does the program print 'Static block runs'?

A Step 3

B Step 2

C Step 4

D Never

💡 Hint

Look at execution_table output column for each step

If we create multiple objects of Test, how many times does the static block run?

A Once

B Once per object

C Twice

D Never

💡 Hint

Refer to key_moments answer about static block execution frequency

Concept Snapshot

Static blocks run once when the class loads.
They initialize static variables.
Run before main or any object creation.
Syntax: static { /* code */ }
Useful for setup tasks.
Runs only once per class load.

Full Transcript

Static blocks in Java run once when the class is loaded by the JVM. This happens before the main method starts or any objects are created. In the example, the static block sets the static variable x to 5 and prints a message. The variable x starts with a default value of 0, then changes to 5 after the static block runs. The main method then prints the value of x. Static blocks are useful to initialize static variables or run setup code. They do not run again if multiple objects are created.

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