Variables, primitive types and printf

Variables are the program’s memory. Every variable has a type, and the type decides both what it can store and how it is printed. Writing 7 instead of 7.0 in an output question, or thinking the result of 'A' + 1 is B, comes from not knowing the rules on this page.

Golden rules:

1. Type first, name second: int age = 19;. You write the type once, then you only use the name.
2. A decimal literal is a double. 2.5 is a double, 2.5f is a float.
3. Small to large is automatic, large to small needs a cast: int → double happens by itself, double → int needs (int).
4. A char is really a number. 'A' + 1 gives an int: 66.
5. In printf, %.2f rounds; the (int) cast truncates.

Declaring and assigning variables

int age;            // declaration
age = 19;           // assignment
int goals = 3, assists = 5;   // declaration + initial value (initialization)
double gpa = 3.25;
char grade = 'B';
boolean passed = true;
System.out.println(age + " " + goals + " " + assists + " " + gpa + " " + grade + " " + passed);
age = age + 1;      // the right side is computed first, then written to age
System.out.println(age);

19 3 5 3.25 B true
20

= is not mathematical equality, it is assignment: compute the value on the right, put it in the box on the left. That is why age = age + 1 makes sense: take the old value, add one, write it back.

Variable name rules: it can contain letters, digits, _ and $, it cannot start with a digit, and it cannot be a reserved word (keyword) such as int or class. By convention it starts with a lowercase letter and words are joined with capital letters: totalScore, studentCount (camelCase).

PITFALL · variable with no value. If you write int count; and then System.out.println(count); without ever giving it a value, it does not compile: variable count might not have been initialized. Local variables inside a method have no default value.

The eight primitive types

Java has eight basic (primitive) types. In the exam int, double, char and boolean appear most; for the others it is enough to know that they exist and their order.

The ranges were taken from Java’s own constants:

System.out.println(Byte.MIN_VALUE + " " + Byte.MAX_VALUE);
System.out.println(Integer.MIN_VALUE + " " + Integer.MAX_VALUE);
System.out.println(Long.MAX_VALUE);

-128 127
-2147483648 2147483647
9223372036854775807

String is not primitive. It is a class, written with double quotes: "Ali". A char uses single quotes and is a single character: 'A'. 'Ali' does not compile.

Overflow

If you go past the limit of int, Java gives no error; the number comes back from the other end:

int m = Integer.MAX_VALUE;
m = m + 1;
System.out.println(m);

-2147483648

A char is a number

Every character has a code (Unicode). Three are enough to memorize: 'A' = 65, 'a' = 97, '0' = 48. When you do arithmetic with a char, the result is an int.

char c = 'A';
System.out.println(c + 1);            // char + int = int
System.out.println((char) (c + 1));   // convert back to char
System.out.println('a' + 'b');        // 97 + 98, two numbers
System.out.println("" + 'a' + 'b');   // String first, then concatenation
char c2 = 'A';
c2++;                                 // ++ stays a char
System.out.println(c2);
System.out.println((int) '0');

66
B
195
ab
B
48

PITFALL · sum of two chars. The result of 'a' + 'b' is 195, not ab. + does addition when neither side is a String. If you want concatenation, put "" at the start.

Type conversion and casting

Widening conversion: from a smaller type to a larger one happens automatically, with no data loss. The order is: byte → short → int → long → float → double.

Narrowing conversion: from larger to smaller. Because data can be lost, the compiler insists that you write a cast. A cast truncates the fractional part, it does not round.

int i = 7;
double dd = i;                  // widening, automatic
System.out.println(dd);
double x = 9.99;
int t = (int) x;                // narrowing, cast required
System.out.println(t);
System.out.println((int) -3.7);                 // truncates toward zero
System.out.println((int) 3.7 + (int) 2.6);      // 3 + 2
System.out.println((int) (3.7 + 2.6));          // (int) 6.3
System.out.println(Math.round(3.7));            // this is how you round

7.0
9
-3
5
6
4

If you do not write the cast:

int x = 2.5 * 2;
float f = 1.5;

Narrow.java:3: error: incompatible types: possible lossy conversion from double to int
        int x = 2.5 * 2;
                    ^
Narrow.java:4: error: incompatible types: possible lossy conversion from double to float
        float f = 1.5;
                  ^
2 errors

Even if 2.5 * 2 = 5.0, its type is double; the compiler looks at the type, not the value.

Formatted output with printf

System.out.printf takes a format string and puts the arguments, in order, in place of the specifiers (format specifiers) that start with %. Unlike println, it does not move to a new line automatically; for a line break you write %n (or \n).

String name = "Deniz";
int score = 87;
double avg = 78.456;
System.out.printf("%s scored %d points%n", name, score);
System.out.printf("Average: %.2f%n", avg);
System.out.printf("Average: %.1f%n", avg);

Deniz scored 87 points
Average: 78.46
Average: 78.5

PITFALL · %.2f rounds, a cast truncates. printf("%.1f", 78.456) → 78.5. But (int) 78.9 → 78. Do not mix the two up.

TIP · the comma on a Turkish computer. printf uses the system’s locale. On a computer set to Turkish, %.2f may print 78,46 with a comma. English is accepted in the exam: write a dot.

Worked example: swap

To exchange the values of two variables you need a third box. Otherwise one of them is lost.

int a = 3, b = 8;
int temp = a;   // save the value of a
a = b;
b = temp;
System.out.println(a + " " + b);

8 3

              a   b   temp
start         3   8   -
temp = a      3   8   3
a = b         8   8   3
b = temp      8   3   3

Common mistakes

int p = 5;
int q = p;
p = 10;
System.out.println(p + " " + q);

System.out.printf("%d-%d%n", 7, 3);

System.out.printf("Total: %.2f%n", 12.0 / 7);

System.out.printf("[%4d][%-4d]%n", 7, 7);

char ch = 'C';
System.out.println(ch + 2);
System.out.println((char) (ch + 2));

System.out.println('x' + "y" + 'z');
System.out.println('x' + 'y' + "z");

double v = 5.9;
int w = (int) v * 2;
int w2 = (int) (v * 2);
System.out.println(w + " " + w2);

int r = 2147483647;
r++;
System.out.println(r);

int k = 4;
double z = k / 8;
double z2 = k / 8.0;
System.out.println(z + " " + z2);

int a = 2.5 * 2;       // line 1
double b = 7;          // line 2
float c = 1.5;         // line 3
long d = 100;          // line 4

int s1 = 1, s2 = 2;
s1 = s2;
s2 = s1;
System.out.println(s1 + " " + s2);

System.out.printf("%s is %d years old and has a %.1f GPA%n", "Ali", 20, 3.456);