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CS205 Notes 3 (28 August 2006)

cs205 Monday 28 August 2006

Assignments Due

Now: Problem Set 1
Wednesday, 30 August: Read Chapter 3 and Chapter 9
Friday, 1 September: Quiz 1 (covers classes 1-4 and Chapters 1-3 and 9)
Monday, 4 September: Read Chapter 4 and Chapter 5 through section 5.2
Friday, 8 September: Problem Set 2 (will be handed out Wednesday)

Java Semantics

What are the two possible meanings of an assignment statement (e.g., a = b) in Java?

Excerpts from Java API documentation:

java.lang.String

The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.

Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared. For example:

     String str = "abc";

is equivalent to:

     char data[] = {'a', 'b', 'c'};
     String str = new String(data);

public boolean equals(Object anObject)

Compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.

public class Strings {
    public static void main (String [] args) {
        String s = new String ("hello");
        String t = new String ("hello");
        StringBuffer sb = new StringBuffer ("he"); 
        StringBuffer tb = sb;
        String s1 = "hello";
        String t1 = "hello";

        sb.append ("llo");
        tb.append (" goodbye!");

        s.concat (" goodbye!");
        t = s.concat (" goodbye!");
        
        // What are the values of s, t, sb and tb now?
        
        // Which of these are true:
        //    a) s == t
        //    b) s1 == t1
        //    c) s == s1
        //    d) s.equals (t)
        //    e) sb == tb
        //    f) t.equals (tb)
    }
}

Specifying Procedures

What kinds of documentation do we need to allow modular development?

How does a specification constrain a client?

Our procedural specifications will use three clauses:

REQUIRES — what the client must satisfy before calling the procedure (the precondition)
MODIFIES — what state may be modified by the procedure (any state not listed in a modifies clause, may not be modified)
EFFECTS — what the procedure guarantees will be true after the procedure returns (the postcondition), so long as the client satisfied the precondition in the REQUIRES clause

An implementation of a procedural specification is satisfactory if: if the client satisfies the precondition before the call, the postcondition is always satisfied after the call returns.

requires → (effects & everything that is not listed in modifies is unchanged)

What is the point of having a MODIFIES clause?

What are the advantages and disadvangates of formal (instead of informal) specifications?

If a procedure's specification has no REQUIRES clause, what is the precondition?

If a procedure's specification has no MODIFIES clause, what can it modify?

What does it mean when a procedure has no EFFECTS clause?

Then you should say what you mean, the March Hare went on.
I do, Alice hastily replied; at least--at least I mean what I say--that's the same thing, you know.
Not the same thing a bit! said the Hatter. You might just as well say that "I see what I eat" is the same thing as "I eat what I see"!
You might just as well say, added the March Hare, that "I like what I get" is the same thing as "I get what I like"!
You might just as well say, added the Dormouse, who seemed to be talking in his sleep,
that "I breathe when I sleep" is the same thing as "I sleep when I breathe"!
It IS the same thing with you, said the Hatter, and here the conversation dropped,
and the party sat silent for a minute, while Alice thought over all she could remember about ravens and writing-desks, which wasn't much.

Alice in Wonderland