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📦 Variables & Types · Beginner

Declaring & initializing variables in Java

Type + name, assignment, definite assignment rule for locals.

🧩 The mysteryDeclare a variable, forget to give it a value, then print it. Does Java print 0? null? Random garbage? The answer protects you from a whole family of bugs.

A labeled box

A variable is a named box that holds one value of one type. Declaring it means writing the type and the name. Initializing it means giving it its first value, often on the same line.

int age;          // declare
age = 30;         // assign
int year = 2025;  // both at once

Pick the right type

int holds whole numbers, double holds decimals, boolean holds true or false, and String holds text. A decimal literal like 9.99 is a double, so an int can't hold it.

int count = 3;
double price = 9.99;
boolean ready = true;
String name = "Ada";
🔮 Predict it

Your turn

total was declared but never given a value. What happens?

int total;
System.out.println(total);
  1. 0
  2. null
  3. Compile error
Show the answer

Compile error: "variable total might not have been initialized". A local variable (one declared inside a method) gets no default value. You must assign it before reading it.

Definite assignment

Before a local variable is read, the compiler checks every possible path through the code. If any path could reach the read without an assignment, it's rejected. An if/else that assigns in both branches passes.

int x;
if (ready) {
    x = 5;
} else {
    x = 7;
}
System.out.println(x);  // ok
🔮 Predict it

Your turn

Does the compiler accept this? What does it print?

int x;
boolean big = false;
if (big) {
    x = 100;
} else {
    x = 1;
}
System.out.println(x);
  1. 1
  2. 100
  3. Compile error
Show the answer

Both branches assign x, so it's definitely assigned before the println. big is false, so the else branch runs and x is 1.

⚠️ The trap

The missing else

If passed is false, score is never assigned, so the compiler rejects the println line. Fix it by initializing up front (int score = 0;) or by adding an else that assigns.

int score;
if (passed) {
    score = 100;
}
System.out.println(score);  // error
💼 In the real world

In real projects

In C, an uninitialized local holds leftover bytes from memory: a source of baffling, random bugs that change from run to run. Java turns that whole category into a compile error, so it never reaches users.

Key takeaways

  1. Declaration: type + name
  2. Initialization: the first assignment, often on the same line
  3. Locals must be definitely assigned before they're read
🤯 Did you know?

The definite-assignment rules fill an entire chapter of the Java Language Specification (Chapter 16). That's how seriously Java takes "never read garbage".

Practice questions

Which type fits the blank?

___ price = 9.99;
  1. int
  2. boolean
  3. char
  4. double
Check your answer

double. 9.99 is a decimal literal, which has type double. An int can't hold it without a cast.

What does this print?

int total;
System.out.println(total);
  1. 0
  2. null
  3. Throws NullPointerException
  4. Compile error
Check your answer

Compile error. total is a local variable that was never assigned. javac reports "variable total might not have been initialized".

Next: Java has exactly eight primitive types. Can you name them all before the next screen does?