static block
- Java course
- Intermediate
- Lesson 32/116
- 5 min
Learn class initialization with static blocks.
static block
A static block runs once when the class is loaded. It is used to initialize static fields; especially for computations done in multiple steps. It is written as static { }.
Example: class Config { static double rate; static { rate = 1.5; } }. Here the static block assigns 1.5 to rate. When the class is loaded, this block runs once; all instances see the same value. A static block is useful when initialization requires more than a simple assignment. If multiple static blocks exist, they run in order from top to bottom.
When to use a static block?
A static block is used when a static field cannot be initialized with a simple value. For example, when a computation, file reading or external source is needed. A static block gathers this complex initialization in one place.
Example: static List<Integer> data; static { data = new ArrayList<>(); data.add(10); data.add(20); }. Here the data list is populated inside the static block. When the class loads, the list is ready. This is a multi-step initialization that cannot be done with a single static List<Integer> data = new ArrayList<>(); line. A static block organizes initialization logic at the class level.
Practice Add a static block to Config
Define a class Config. Add a static int base field (no initial value). Add a static block; its body runs base = 100;. In main print the value of Config.base.
Starter code
// Define the Config class
// Add a static base field and a static block
// Print the base value in mainSample solution
class Config {
static int base;
static {
base = 100;
}
public static void main(String[] args) {
System.out.println(Config.base);
}
}Expected output
100
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When does a static block run?
- Every time an instance is created
- Once when the class is loaded Correct answer
- When a method is called
- When the program ends
Explanation: A static block runs once when the class is loaded.
2 Complete the static block
class Config {
static double rate;
____ {
rate = 1.5;
}
}class Config {
static double rate;
static {
rate = 1.5;
}
}Explanation: A static block runs during class loading.
3 Match the static block parts
| Term | Meaning |
|---|---|
static { } |
Block syntax |
class load |
Run time |
rate = 1.5 |
Initialization |
once |
Run frequency |