Each java primitive type has a corresponding wrapper class. Constructing a wrapper class creates an object. You don't have to construct a wrapper class before using, because java support auto-boxing -- wherever a method or constructor expect a wrapper class, you can pass in the corresponding primitive type data, java will automatically convert the primitive type into the wrapping class.
Let's take a look at examples.
auto-boxing
OCAJP>cat test.java
import java.util.*;
class test {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>(); //list expect Integer object
list.add(1); //auto boxing, java auto convert int 1 to new Integer(1);
list.add(new Double(1.1).intValue());
for(long j : list) { //auto unboxing, convert Integer to int, then type cast to long
System.out.println(j);
}
for(Integer k : list) {
System.out.println(k + 1.1f); //auto unboxing to int, then promote to float
}
}
}
OCAJP>javac test.java
OCAJP>java test
1
1
2.1
2.1
Wrapper constructor and functions.
new Type()
<type>Value()
parse<Type>()
OCAJP>cat test.java
class test {
public static void main(String[] args) {
Boolean b = new Boolean(true);
boolean b1 = b.booleanValue();
//boolean b2 = Boolean.parseBoolean(b1); //boolean cannot be converted to String
boolean b3 = Boolean.parseBoolean("true");
Boolean b4 = Boolean.valueOf("true");
Boolean b5 = Boolean.parseBoolean("tree");
System.out.println("b5 = " + b5);
Byte bt = new Byte((byte)1);
//Byte bt2 = new Byte(1); //possible lossy conversion from int to byte
Byte bt3 = new Byte("1");
Byte bt4 = new Byte(Byte.parseByte("1"));
Byte bt5 = Byte.valueOf("1");
Byte bt6 = Byte.valueOf((byte)1);
//Byte bt6 = Byte.valueOf(1); //possible lossy conversion from int to byte
byte bt7 = Byte.parseByte("1");
byte bt8 = bt6.byteValue();
short s0 = bt6.shortValue();
int i0 = bt6.intValue();
//Byte bt9 = Byte.valueOf("1.999");//java.lang.NumberFormatException
long l0 = bt6.longValue();
float f0 = bt6.floatValue();
double d0 = bt6.doubleValue();
System.out.println("d0=" + d0);
Short s = new Short((short)1);
short s1 = s.shortValue();
byte b9 = s.byteValue();
Integer i = new Integer(1);
//System.out.println(new Integer(null)); //java.lang.NumberFormatException: null
Long l = new Long(1);
Float f = new Float(1);
Double d = new Double(1);
System.out.println("d="+d);
Double d1 = new Double(i);
Double d2 = new Double(d1);
Character c = new Character('A');
char c1 = c.charValue();
Character c = new Character('A');
char c1 = c.charValue();
}
}
OCAJP>javac test.java
OCAJP>java test
b5 = false
d0=1.0
d=1.0
