Statements and Expressions

In general, there is no comma expression.

Consider the following C program which contains a comma expression.

#include < stdio.h>
int main(){
int t;
int i=2;
int j=4;
t=(i+=2,j==i);
printf("%i\n",t);
return 0;
}

Running it produces 1 as output.

However, compiling

class Testit{
public static void main(String args[]){
boolean t =true;
int i=2;
int j=4;
t=(i+=2,j==i); }
}
Produces

C:\JAVA> javac Testit.java
Testit.java:6: ')' expected.
t=(i+=2,j==i)

However, one can use comma's for sequential execution within the control expressions of a for statement.

In particular,

public class T1{
    public static void main(String args[]){
        int i,j;
        for(i=1,j=2;(i+j)< 10;i++,j++)
    System.out.println(new Integer(i+j).toString());
    }
}

compiles and produces

C:\drop> java T1
Symantec Java! JustInTime Compiler Version 210.054 for JDK 1.1.2
Copyright (C) 1996-97 Symantec Corporation

3
5
7
9

The control expression of a conditional statement must be boolean valued.

In particular, gone is everyone's favorate run time bug, using = in place of ==.

Valid C code such as

int a;
a=expression;
if(a = 1) /* it is now! */
{ something}

produces a Java compile error.

---

The goto statement has been eliminated.

The role of the unrestricted goto has been controversial for some time. On one hand it can be misused to create "spaghetti logic" in programs. At the same time there are situations where it provides the ideal programming solution to a flow of control problem. Having removed the goto Java provides alternatives to handle situations where it would provide a suitable solution.

• The try-throw-catch mechanism provides a solution in situations where "long jumps" are required. This mechanism will be discuss in detail.
• "Intermediate jumps" out of blocks of code are handled with labeled break and continue statements which are straight-forward extensions of the usual unlabeled versions. Here are examples.

  First a class that distinguishes the actions associated with a labeled continue from those of an unlabeled break

  class ContinTest{
      public static void main(String args[]){
      boolean t=true;
      int i =1,j=0;
          b:while(j< 5){
                  i=1;j++;
                  System.out.println("The times table for "+j);
                 while(true){
                    System.out.println(i*j);
                    if(i==5){
                          if(j < 5)
  			    continue b;
                          else 
  			    break;
                    }//if
                  i++;
                  }//while   
              System.out.println("all done");
          }//b
     }//main
  }//class
  
  

  Here is the output. Note that the "all done" only executes on break

  /siegel/home/public_html%java ContinTest 
  The times table for 1
  1
  2
  3
  4
  5
  The times table for 2
  2
  4
  6
  8
  10
  The times table for 3
  3
  6
  9
  12
  15
  The times table for 4
  4
  8
  12
  16
  20
  The times table for 5
  5
  10
  15
  20
  25
  all done
  
  

Next a labeled break statement.

class BreakTest{
    public static void main(String args[]){
    boolean t=true;
    a:{
        b:{
            c:{
                System.out.println("before");
               if(t)break b;
               }//c
            System.out.println("will not execute");
        }//b
        System.out.println("next to execute");
    }//a
   }//main
}//class

Here is the output.

C:\JAVA> java BreakTest
before
next to execute

At first glance, it appears that labeled breaks and continues can be used to simulate a goto, however, there are restrictions on their use as illustrated by the following output which results from removing the if clause from the statement containing the break

 BreakTest.java:11: Statement not reached.
            System.out.println("will not execute");
            ^
1 error

Interestingly, if you insert the line

if(true) break b;

The program compiles and runs as before. More on this point later.