Wrapper classes

Data Types and Exceptions in Java

Jim White

Java Developer

Java 8 primitives

The 8 Java primitives are the basic data types from which all data is constructed.

Data Types and Exceptions in Java

Wrapper classes

  • Each primitive type has an equivalent wrapper class
primitive Wrapper Class
byte Byte
short Short
int Integer
long Long
float Float
double Double
boolean Boolean
char Character

Wrapper classes allow primitives to be treaded like an object. Wrapper classes are subclasses of Object.

Data Types and Exceptions in Java

Wrapping primitives

  • Instances of a wrapper class hold a single primitive
    • Example: Integer holds a single int primitive value

Wrapper objects contain a primitive as their value

Data Types and Exceptions in Java

Wrapper details

  • Wrapper classes have additional fields and methods
    • Fields (like max value) about the type
    • Methods to work with the wrapped primitive type
  • Wrapper classes are in base language
    • Use them without import (more on imports later)
Data Types and Exceptions in Java

Creating wrapper objects

  • Assign a primitive to a variable of the wrapper type
    • Syntax: Wrapper-type variable = primitive-value;
Integer age = 12;
Double cost = 150250.55;
Float interest = 5.5f;
Character grade = 'A';
Boolean isActive = true;
  • Wrapper objects can also have no value (null)
Integer age = null;
Data Types and Exceptions in Java

Using wrapper objects

Integer age = 12;
  • Print them
    System.out.println(age);    // Displays 12
  • Get their primitive value
    int x = age.intValue();     // x is assigned 12
  • Perform other operations on them
    double z = age.doubleValue();  //z is 12.0
String y = age.toString();  // y is assigned "12"
Integer teenAge = 16;
int smaller = age.compareTo(teenAge);  // smaller is -1 since 12 < 16
Data Types and Exceptions in Java

Wrapper Static Methods

  • Wrapper classes come with static methods
  • Used to perform operations on the associated primitives
    int x = Integer.sum(8,12);  // x is 20
int y = Integer.remainderUnsigned(102, 10);  // y is 2
  • Used to convert between String and primitive
    int z = Integer.parseInt("123");   // z is 123;
boolean ans = Boolean.parseBoolean("false");  // ans is false
1 See https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html for more on the Integer wrapper
Data Types and Exceptions in Java

Wrapper Static Fields

Example static fields on wrapper classes

System.out.println(Integer.MAX_VALUE); // Maximum value an int can have
System.out.println(Integer.MIN_VALUE); // Minimum value an int can have

2147483647
-2147483648

System.out.println(Boolean.TRUE); // Corresponding to the primitive value of true
System.out.println(Boolean.FALSE); // Corresponding to the primitive value of false

true
false
Data Types and Exceptions in Java

Wrapper Static Fields

Example static fields on wrapper classes

System.out.println(Character.SPACE_SEPARATOR);  // Unicode for the regular space like ' '
System.out.println(Character.LINE_SEPARATOR); // Unicode for line return like '\n'

12
13
Data Types and Exceptions in Java

Interesting Wrapper Methods

Wrapper Method Returns
Boolean.logicalAnd(boolean a, boolean b) boolean
Boolean.logicalOr(boolean a, boolean b) boolean
Boolean.parseBoolean(String s) boolean
Character.getNumericValue(char ch) Unicode int value of the char
Character.isDigit(char ch) boolean
Character.isLowerCase(char ch) boolean
Character.isWhitespace(char ch) boolean
Double.parseDouble(String s) double
Double.longValue() the Double value, rounded down, as a Long
Data Types and Exceptions in Java

Why Wrappers?

  • Primitives have no methods (only operations)
    • Wrapper classes come with useful methods
      int score = Integer.parseInt("8");
  • Wrappers allow primitives to be used as objects; like putting them in a collection
    • Learn about collections in the Chapter 2.
Data Types and Exceptions in Java

Wrappers can be null

  • Wrappers allow instance or static variables of the type to be null
    • Primitive instance or static variables have a default value when not initialized
int herAge; // age is 0 by default
Integer hisAge = null;
if (hisAge != null) {
  // do something when hisAge is not set
}
Data Types and Exceptions in Java

Let's practice!

Data Types and Exceptions in Java

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