1.4 Assignment Statements and Input

1. Giving a variable its first value

Every variable must be assigned a value before it can be used in an expression, and that value must be of a compatible data type. A variable is initialized the first time it is assigned a value 1.4.A.1.

// does not compile
int n;
System.out.println(n);

Real view: the compiler rejects the code because n has no value yet.

error: variable n might not have been initialized

Declaring and initializing can happen in one statement or in two:

int lives = 3;     // declared and initialized together
int bonus;         // declared only
bonus = 2;         // initialized here
System.out.println(lives + bonus);

Output:

5

A reference variable can be assigned a new object, or null if there is no object. The literal null is a special value that means a reference is not associated with any object 1.4.A.1.

String title = "Chess Club";
String nickname = null;      // no String object yet
System.out.println(title);
System.out.println(nickname);

Output:

Chess Club
null

Printing a null variable prints the word null. Calling a method on it is different: that throws a NullPointerException (topic 1.14).

2. The assignment operator

The assignment operator = gives a variable its first value (initializes it) or replaces the value it holds. Java stores the value of the expression on the right side in the variable on the left side 1.4.A.2.

Java evaluates the right side first, using the current values, and then stores the result. So c = c + 1; is not an equation: it adds 1 to c.

int c = 4;
c = c + 1;
System.out.println(c);

Output:

5

Assigning a primitive copies the value. After int b = a;, changing a does not change b.

int a = 10;
int b = a;   // b gets a copy of 10
a = 99;
System.out.println(a + " " + b);

Output:

99 10

For a reference variable, the value copied is the reference, so both variables then refer to the same object (see aliasing in topic 1.14).

Outside the course: assignments inside expressions, such as a = b = 4;.

3. Expressions produce one value

During execution, an expression is evaluated to produce a single value, and that value has a type based on the evaluation 1.4.A.3. a + 4 is an expression: it produces a value. A simple statement, such as an assignment or a method call, is a complete instruction that ends in ;, such as int b = a + 4;.

int a = 7;
double half = a / 2.0;  // the expression a / 2.0 has type double
int whole = a / 2;      // the expression a / 2 has type int
System.out.println(half + " " + whole);

Output:

3.5 3

The type of the expression must be compatible with the variable. a / 2.0 is a double, so it could not be stored in an int variable without a cast (topic 1.5).

4. Trace tables

A trace table follows the values step by step. Each row runs one line of code and shows the values stored afterwards. A blank cell means the variable does not exist yet.

int a = 3;
int b = a + 4;
a = b * 2;
b = b - a;
System.out.println(a + " " + b);

Line	Code	a	b
1	int a = 3;	3
2	int b = a + 4;	3	7
3	a = b * 2;	14	7
4	b = b - a;	14	-7

Output:

14 -7

Watch out: line 4 uses the NEW value of a (14, not 3). Update your trace table after every line; never use a value from memory.

5. Input

Input can come in a variety of forms, such as tactile (touch), audio, visual or text. The Scanner class is one way to get text input from the keyboard 1.4.B.1. No specific form of user input is tested on the AP® exam, so you only need the idea.

import java.util.Scanner;

public class Greeting
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.println("Name?");
        String name = input.nextLine();
        System.out.println("Age?");
        int age = input.nextInt();
        System.out.println("Hi " + name + ", next year you will be " + (age + 1));
    }
}

Real view: the console when the user types Ana and 16 (the typed lines are the user’s input).

Name?
Ana
Age?
16
Hi Ana, next year you will be 17