Static blocks in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a static block in Java?

A static block is a block of code inside a class that runs once when the class is loaded. It is used to initialize static variables or perform setup tasks.

beginner

When does a static block execute in Java?

A static block executes only once when the class is first loaded into memory, before any objects are created or static methods are called.

intermediate

Can a class have multiple static blocks? If yes, in what order do they execute?

Yes, a class can have multiple static blocks. They execute in the order they appear in the source code, from top to bottom.

advanced

What happens if an exception occurs inside a static block?

If an exception occurs inside a static block and is not caught, it causes an ExceptionInInitializerError, preventing the class from loading properly.

intermediate

Why use static blocks instead of initializing static variables directly?

Static blocks allow complex initialization code that can't be done in a single statement, such as loops, conditionals, or try-catch for error handling.

When is a static block executed in Java?

A Every time an object is created

B When the class is loaded

C When a method is called

D When the program ends

Static blocks run once when the class is loaded into memory.

Can a Java class have more than one static block?

A No, only one static block is allowed

B Yes, but they execute randomly

C No, static blocks are not allowed in Java

D Yes, and they execute in the order they appear

Multiple static blocks are allowed and run in the order they appear in the code.

What happens if an exception is thrown inside a static block and not caught?

A The program ignores it

B The exception is converted to a runtime exception

C The class fails to load with ExceptionInInitializerError

D The exception is logged but the program continues

Uncaught exceptions in static blocks cause ExceptionInInitializerError and prevent class loading.

Which of these is a good reason to use a static block?

A To initialize static variables with complex logic

B To initialize instance variables

C To execute code every time an object is created

D To replace constructors

Static blocks are used for complex static variable initialization.

Static blocks run before which of the following?

A All of the above

B Before main method runs

C Before any object is created

D Before any static method or variable is accessed

Static blocks run once when the class loads, before main, static methods, or object creation.

Explain what a static block is and when it runs in Java.

Think about when the class is first used by the program.

Describe how multiple static blocks behave in a Java class.

Consider the order of code in the class file.

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