Java 2 API: java.lang

System properties: http://java.sun.com/docs/books/tutorial/essential/system/properties.html

Wrapper Classes for Primitive Types

java.lang contains some of the core classes used in Java. Specifically, it contains the Object super class discussed in the previous lecture. Also, it contains object wrappers for all of the primitive types: Boolean, Byte, Character, Double, Float, Integer, Long. These objects, once created as immutable. In general "value" objects are immutable in Java. To create one of these objects, you pass a value using a primitive type to its constructor, e.g.

Float aFloat = new Float(1.3);

To extract the primtive type, there is generally a "xValue" method, e.g.

float val = aFloat.floatValue();

In addition to being useful for storing primitive types in collections, these wrapper classes also contain a variety of useful utility methods.

Math

The Math class contains the kinds of operations found in scientific calculators as well as some other goodies, e.g.

Math.abs() computes the absolute value of a number
Trigonometric funtions: Math.sin(), Math.cos(), Math.tan(), Math.asin(), Math.acos(), Math.atan(), Math.toDegrees(), Math.toRadians(), Math.floor(), Math.ceil(), Math,exp(), Math.pow(), Math.min(), Math.max(), Math.random(), Math.sqrt()

Runtime

Runtime.getRuntime() returns a Runtime object. Runtime can be used for a few different things. Among other, you can use Runtime to execute external programs from Java using the exec() function. You can also use it to find out things about your environment, such as the amount of memory used and the total amount of available memory.

System

System contains a grab-bag of system-related functions. For example, System.currentTimeMillis() returns the current time in milliseconds since Jan 1, 1970 UTC.You can use this value to generate Date and Calendar objects. System.getProperty() returns a variety of useful properties (see link at top of page). System.getSecurityManager returns the security context -- though this is usually configured using policy files.

String and StringBuffer

String is the class normally used for String manipulation. String is a value object, and is therefore immutable. Thus, the following code produce a lot of temporary objects:

String hello = "h" + "e" + "l" + "l" + "o";

This expression probably creates 9 temporary objects which are immediately garbage collected. StringBuffer is designed to let you build strings without creating so many temporary objects, e.g.

StringBuffer helloBuffer = new StringBuffer();
helloBuffer.append("h");
helloBuffer.append("e");
helloBuffer.append("l");
helloBuffer.append("l");
helloBuffer.append("o");
String hello = helloBuffer.toString();