Java Data Types

The primitive types

Java has eight built-in primitive types. In everyday code you reach for four of them:int, double, boolean, and char. The rest matter when size or precision does.

public class Main {
  public static void main(String[] args) {
    int n = 7;
    double x = 3.5;
    char grade = 'B';
    boolean ok = true;
    System.out.println(n + " " + x + " " + grade + " " + ok);
  }
}

String is a class, not a primitive

String holds text in double quotes. It is a full object with methods like length()and toUpperCase(). Join strings with +.

public class Main {
  public static void main(String[] args) {
    String name = "Ada";
    System.out.println(name.length());        // 3
    System.out.println(name.toUpperCase());   // ADA
  }
}

public class Main {
  public static void main(String[] args) {
    String a = "hi";
    String b = new String("hi");
    System.out.println(a == b);        // false  (different objects)
    System.out.println(a.equals(b));   // true   (same text)
  }
}

Integer division and casting

Dividing two ints throws the fraction away — 17 / 5 is 3, not3.4. Make one side a double to keep the decimals. Going the other way,(int) drops the fraction on purpose.

public class Main {
  public static void main(String[] args) {
    System.out.println(17 / 5);      // 3    (int division truncates)
    System.out.println(17 / 5.0);    // 3.4  (one double keeps decimals)
    System.out.println((int) 9.8);   // 9    (cast drops the fraction)
  }
}

Wrapper types and overflow

Each primitive has a matching object type — Integer for int, Double fordouble — used when you need objects (for example inside an ArrayList). They also carry helpers like Integer.parseInt("42") to turn text into a number.

Key Takeaways

• Use int, double, boolean, char for everyday values; long for big numbers.
• String is a class — compare text with .equals(), never ==.
• Int-by-int division truncates; make one side a double to keep the fraction.
• int overflows silently past ~2.1 billion — use long when values can grow.