Java Language Reference

CONTENTS

1. Input/Output
2. Classes
3. Interface
4. String Class
5. Arrays
6. Casting
7. References
8. Bottom of Page

Input/Output:

• System has 3 static members: in, out, err. Normally, err defaults to the screen (as does "out"). But you can redirect it as follows:

  	//============================================
  	
  	System.err.println("This goes to the screen");
  	FileOutputStream errFile = new FileOutputStream("err.log");
  	System.err = new PrintStream(errFile, true);
  	System.err.println("This goes to err.log file");
  	
  	//============================================
  	

• Basic output is usually accomplished using 2 functions:

  	//============================================
  	
  	System.out.print(...)
  	System.out.println(...)
  	
  	//============================================
  	

  Almost anything can be placed as argument for these print functions. In particular, every object x has a "print value", which is a String: this is simply obtained by the x.toString() method available with all objects. Useful note: you can concatenate these print values by "+". E.g. System.out.print("n = " + n);
• DataInputStream Class: "ReadStreams" is to be attached like a "filter" to the backend of an input stream. They are intended for reading values of primitive types:

  	  //========================================================
  	
  	  DataInputStream r = new DataInputStream(); // by default, r
  		// is attached to System.in.  If you want to read
  		// from a file named "foo.txt", then say instead:
  		// new DataInputStream(new FileInputStream("foo.txt"))
  	  int i;
  
  	  for (r.skipWhite(); !r.eof(); r.skipWhite())
  	  {  i = r.readInt();
  	     System.out.println(i);
  	  }
  	  r.close();
  
  	Other methods:
  		r.readChar()  -- returns next char
  		r.readBoolean() 
  		r.readLong()
  		r.readFloat()
  		r.readDouble()
  		r.readString()
  		r.readLine()  -- returns String
  		r.readln()    -- returns void
  		r.eof()	      -- returns true if no more data in r
  		r.eoln()      -- returns true if no more data in line
  
  	
  	  //=========================================================
  	

  To make the default input for ReadStream to be XXX, say ReadStream.setIn(XXX) where XXX is a InputStream.
• FileInputStream API.
• FileOutputStream API.
• String Reader API.
• String Writer API.

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Classes:

• First declare a class:

  	  //=====================================
  	
  	  class C1 {
  	    // member variables
  	    int myName;
  
  	    // constructors
  	    C1(int n) {...}
  
  	    // methods
  	    public void doThis() {...}
  	  }
  	
  	  //=====================================
  	

• Next instantiate it (with objects):

  	  //=====================================
  	
  	  C1 C1object;
  	  C1object = new C1(5);
  	
  	  //=====================================
  	

• Subclassing (or inheritance):

  	  //=====================================
  	
  	  class C2 extends C1 {...}
  	
  	  //=====================================
  	

• Every class is considered a subclass of java.lang.Object. Moreover, a class with the "final" modifier cannot be subclassed.
• Overriding (a quirk): Any re-definition of a method will OVERRIDE the parent method definition. However, re-definition of member variable does NOT override! For instance, if C2 also defines a member named "myName", then any C2 objects actually has two "myName" variables. Just saying "myName" for C2 will get this->myName. To get the "myName" for C1, you will need to use super->myName. If there are more than two layers of "myName", then I suppose we need to do "super->super->...?"

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Interface:

• WHY? Subclassing is not always adequate relationship among classes. We need a new concept...
• Syntax:

  	  //=====================================
  	
  
  	  interface LIST {
  	  ...
  	  }
  
  	  class MyList implements LIST {
  	  ...
  	  }
  
  	  class AugList extends MyList implements LIST {
  	  ...
  	  }
  	
  	  //=====================================
  	

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Strings:

• By definition, Strings are immutable. If you need a more flexible concept, use StringBuffer (in java.lang also) instead.
• You can use an array of char or array of bytes to construct a string:

  	  //=====================================
  	
  	  char c[] = {'a', '1', 'b', '2'};
  	  String s = new String(c);	// s = "a1b2"
  	
  	  //=====================================
  	

• There is a special operator for Strings: "+" is considered as String concatenation:

  	  //=====================================
  	
  		String s = "do"		// s = "do"
  		String ss = s + s;	// ss = "dodo"
  	
  	  //=====================================
  	

• Other useful String operations:

  	  //=====================================
  	
  	  String s = "good";		// instantiation
  	  int i = s.length;		// i = 4
  	  boolean b = s.equals("GOOD"); // b=false
  	  b = s.equalsIgnoreCase("GOOD"); // b=true
  	  i = s.indexOf('o');  	        // i=1 (first occurrence)
  	  i = s.lastIndexOf('o');	// i=2 (last occurrence)
  	  i = s.indexOf("od");		// i=2
  	  String t = s.substring(2);	// t="od"
  	  String tt = s.substring(0,2);	// t="go"
  	  String u = s.concat(" work");	// u="good work"
  	
  	  //=====================================
  	

• More? Click Me

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Arrays:

• Declaration:

  	  //=====================================
  	
  	  int x[] = new int[50];	// array of 50 ints
  	  public int table()[];		// table() returns an int array
  	
  	  //=====================================
  	

• Alternatively:

  	  //=====================================
  	
  	  int[] x = new int[50];
  	  public int[] table();		// looks neater!
  	
  	  //=====================================
  	

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Casting:

• Casting among siblings is not allowed You cannot directly cast a parent to a child (But a parent that was casted from a child can be recasted back to the child)
• Initialization:

  	  //=====================================
  	
  	  char[] c = {'a', 'b', 'c'};
  	  int[] x  = {1,2,3};
  	
  	  //=====================================
  	

• Multidimensions:

  	  //=====================================
  	
  	  char student[][] = new char[10][30];	// each sublist of length 30
  	  int varLists[][] = new int[3][];
  	  varLists[0]      = new int[10];	// each sublist has different
  	  varLists[1]      = new int[20];	//   lengths!
  	  varLists[2]      = new int[5];
  	
  	  //=====================================
  	

• REMARK: arrays carries some surprises when one thinks that Java has only passing parameters by value. See book of [TTD] p.168.

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REFERENCES:

• [TTD] "Java Primer Plus" by P.M.Tyma, G.Torok and T.Downing.

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