Static Block

A shop sets up its price list once, when it opens in the morning, not every time a customer walks in. A static block does that for a class: it's code that runs exactly once, when the class is first initialized, to prepare static data such as lookup tables, constants that need calculation, or settings.

Set Up Once

import java.util.HashMap;
import java.util.Map;
 
public class States {
    static class StateCodes {
        static final Map<String, String> NAMES = new HashMap<>();
 
        static {
            NAMES.put("TS", "Telangana");
            NAMES.put("MH", "Maharashtra");
            NAMES.put("KA", "Karnataka");
            NAMES.put("TN", "Tamil Nadu");
            System.out.println("State codes loaded: " + NAMES.size());
        }
 
        static String name(String code) {
            return NAMES.getOrDefault(code, "Unknown");
        }
    }
 
    public static void main(String[] args) {
        System.out.println("Program started");
        System.out.println(StateCodes.name("TS"));
        System.out.println(StateCodes.name("KA"));
    }
}

Program started
State codes loaded: 4
Telangana
Karnataka

How it works

1. Lines 8 to 14: static { ... } is a static block. It fills the shared map once.
2. "Program started" prints first: the class isn't initialized until it's first used, on line 23.
3. The block runs once. The second call to name on line 24 doesn't run it again.

Map and HashMap store key-value pairs; The collections lessons, later in the course, cover them.

Try it yourself: delete lines 23 and 24. Does "State codes loaded" still print?

When Is a Class Initialized?

A class's static fields are set and its static blocks run the first time the program:

Several static blocks and static field initializers run top to bottom, in the order they appear.

Static Block vs Instance Block

Common Mistakes

Expecting it to run when the program starts. It runs when the class is first used.

Using instance fields in it. There's no object; only static members are available.

Putting risky work in it. A failure breaks the whole class for the rest of the program.

Interview Corner

Practice

1. Easy: Use a static block to fill a static array with the squares of 1 to 10, and print squares[7].
2. Harder: Write two classes, each with a static block that prints its name. Use only one of them in main. Which message prints?

Key Takeaways