Java Doc for Expression.java in  » Scripting » InstantJ » instantj » expression » Java Source Code / Java DocumentationJava Source Code and Java Documentation

An Expression is a piece of Java-code that evaluates to a result of a primitive- or Object-type. Therefor it doesn't have any return-statements and does not end with ';'. Think of an expression as something you can assign to a variable like this

 Object foo = theExpression;
 

 "Hello World"
 account.balance - 100
 2 * Math.PI
 count--; count<0
 

A java-expression can evaluate to a Object- or primitive-type. In the latter case the evaluation of the Expression-object will wrap the primitive result in a Java-type (e.g. Integer).

This type not only wraps the expression's body but alos its environment properties. The properties are kept in a map of name/Class that keeps track of those the expression can use.(e.g. String.class, Account.class or Integer.TYPE). The expression can reference properties in its environment as local variables (see above).

Once you have an instance of an expression you can evaluate it in the environment of property values that you provide. The values are mapped to the property-names and assigned to the expression's local variables before execution.

Example 1:

 // Our expression body
 String body = " 3*4 ";
 // Creating the Expression
 Expression expression = new Expression(body);
 // Evaluate it
 Object result = expression.evaluate();
 

(java.lang.Integer) 12

Example 2:

 // The properties we want to use inside the expression
 Hashtable properties = new Hashtable();
 properties.put("count", Integer.TYPE);
 properties.put("msg"  , String.class);
 // Our expression body
 String body = " count++; msg+new java.util.Date() ";
 // Creating the Expression
 Expression expression = new Expression(body,properties);
 // The values we give to the evaluation	
 Map values = new HashMap();
 values.put("count", new Integer(3));
 values.put("msg"  , "Guybrush was here on " );
 // Evaluate it
 Object result = expression.getInstance(values).evaluate();
 

(java.lang.String) Guybrush was here on Dec 17 11:33:31 EST 2000

 // Evaluate it
 Object result = expression.getInstance().set("count", new Integer(3)).set("msg", "Guybrush was here on").evaluate();