Stacks and compilers - Data Structures and Problem Solving Using Java - page 468

we have the general operator case, which is succinctly described by the code
in lines 27-31.
A simple main routine is given in Figure 11.22. It repeatedly reads a line
of input, instantiates an Evaluator object, and computes its value.

11.2.4 expression trees

In an expression
tree, the leaves
contain operands
and the other
nodes contain
operators.

Figure 11.23 shows an example of an expression tree, the leaves of which are
operands (e.g., constants or variable names) and the other nodes contain
operators. This particular tree happens to be binary because all the operations
are binary. Although it is the simplest case, nodes can have more than two
children. A node also may have only one child, as is the case with the unary
minus operator.

1 /**
2 * Simple main to exercise Evaluator class.
3 */
4 public static void main( String [ ] args )
5 {
6 String str;
7 Scanner in = new Scanner( System.in );
8
9 System.out.println( “Enter expressions, one per line:” );
10 while( in.hasNextLine( ) )
11 {
12 str = in.nextLine( );
13 System.out.println( “Read: “ + str );
14 Evaluator ev = new Evaluator( str );
15 System.out.println( ev.getValue( ) );
16 System.out.println( “Enter next expression:” );
17 }
18 }

figure 11.22
A simple main for
evaluating
expressions
repeatedly

figure 11.23
Expression tree for
(a+b)*(a-b)

*

_

+

a a

b b