Wrapper notes

java.lang

Provides classes that are fundamental to the design of the Java programming language.
Here are some classes in java.lang
- System : The System class contains several useful class fields and methods. It cannot be instantiated.
  Among the facilities provided by the System class are standard input (System.in), standard output (System.out), and error output streams (System.err); access to externally defined "properties"; terminate currently running Java Virtual Machine(System.exit(int)); get the time (currentTimeMillis()): returns the current time in milliseconds starts from 1970 1/1.
- Object class.
- String, StringBuffer class to handle String
- Math : contains all the math-related constants and methods.
- Class class to get information for a particular class.
- Thread class.
- Boolean, Byte, Character, Double, Float, Integer, Long, Shorttype-wrapper classes. The classes correspond to the primitive classes.
java.lang is automatically imported.

In java.long we have classes Boolean, Byte, Character, Double, Float, Integer, Long, Short. They correspond to the primitive classes

boolean, byte, char, double, float, 
int, long, short

respectively.

You can store a value of primitive type in a type-wrapper object whenever an object is required. Example the Vector class can hold only objects, if you want to put numbers in a vector, you wrap each value in a type-wrapper object and provide that object to the vector.
The classes define useful variables, such as MIN_VALUE and MAX_VALUE. The classes also define useful methods for converting values to other types, for converting to strings and so on.

We will take class Double as an example.
It extends Number class which has the following methods

Member	Description
`byte byteValue(), short shortValue(), int intValue(), long longValue(), float floatValue(), double doubleValue()`	Convert the value of this number object to the primitive date types of `byte, short, int, long, float` and `double`

Constructors or members	Description
`static double MAX_VALUE, static double MIN_VALUE`	max/min value
`static double NaN`	Not a number
`static double NEGATIVE_INFINITY, static double POSITIVE_INFINITY`.
`public Double(double value), public Double(String s) throws NumberFormatException`	meaning is clear
`String toString(), static String toString(double d)`	Returns a String representation.
`static double parseDouble(String s) throws NumberFormatException, static Double valueOf(String s) throws NumberFormatException`	Returns a new double(or Double) initialized to the value represented by the specified String.
`boolean equals(Object)`	determine if the object is equal to the Double
`int compareTo(Double anotherDouble)`	Compares two Doubles numerically. The value is bigger then 0 if the `Double` is bigger than `anotherDouble` etc.

Refer to the java document. They are similar to the previous classes. You are required to memorize the common members for the final exam.

What is the difference between valueOf and parseDouble?
Use a short code to find the max possible value of long.
What is the advantage of using wrapper-type classes?
1. How can you change the double d = 3.15 to a Double?
2. How can you change it to a float?
3. Can you change it to int and how?
4. How can you change it to a String ?

valueOf returns the class Double.
parseDouble returns the primitive type double.
System.out.println(Long.MAX_VALUE);
Refer to the beginning of the notes.
1. use Double d2 = new Double(3.15) to change it to a Double.
2. Use (float)3.15 (casting) or d2.floatValue().
3. Yes. Use (int)3.15 (casting) or d2.intValue().
4. d2.toString() or Double.toString(d)