Default Values of Data Types

When a new bank account is opened, the balance starts at zero. Nobody types the zero in; it's the sensible starting value. Java does the same for some variables: if you don't give a field or an array element a value, Java fills in a default value. But for local variables inside a method, it deliberately refuses, and that difference trips up many beginners.

Who Gets a Default, and Who Doesn't

The Default for Each Type

See the Defaults

public class Ticket {
    int seat;
    double fare;
    boolean paid;
    char coach;
    String passenger;
 
    public static void main(String[] args) {
        Ticket t = new Ticket();
        System.out.println("seat: " + t.seat);
        System.out.println("fare: " + t.fare);
        System.out.println("paid: " + t.paid);
        System.out.println("coach code: " + (int) t.coach);
        System.out.println("passenger: " + t.passenger);
 
        int[] counts = new int[3];
        System.out.println("counts: " + java.util.Arrays.toString(counts));
    }
}

seat: 0
fare: 0.0
paid: false
coach code: 0
passenger: null
counts: [0, 0, 0]

How it works

1. Lines 2 to 6 declare five fields without giving them values.
2. Line 9 creates a Ticket object, and Java fills every field with its default.
3. Line 13 casts the char to int to show its code, 0. Printed as a character, '\u0000' is invisible.
4. Line 16 creates an array of three ints. Each element starts at 0, and Arrays.toString on line 17 prints them all.

Try it yourself: add a field long pnr; and print it. Then make the array a String[] instead of int[]. What do the elements start as?

Why Locals Get No Default

Inside a method, Java insists that you assign a variable before reading it:

Errs6.java:4: error: variable x might not have been initialized
        System.out.println(x);
                           ^

This is on purpose. A local variable that you forgot to set is almost always a bug. A silent 0 would hide it; a compile error points straight at it. Fields are different: an object may legitimately be created before all its fields are known, so Java gives them safe starting values.

Common Mistakes

Expecting a local int to start at 0. It doesn't compile. Write int total = 0;.

Comparing a default char with a space. '\u0000' is not ' '. The space character is code 32.

Assuming a new String[] contains empty strings. Its elements are null, not "".

Practice

1. Easy: Write the default value for each: an int field, a boolean array element, a String field, a double static variable, a local int.
2. Harder: Create a boolean[] seatsBooked = new boolean[5];, mark seats 1 and 3 as booked, and print all five values.

Key Takeaways