java for abap programmers 7

7/28/2019 Java for ABAP Programmers 7

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Alistair C. Rooney 2002 All rights reserved

JavaTM for ABAP programmers Lesson 7

Lesson 7 Strings with Java

Dealing with text in some way, shape or form is fundamental to any programminglanguage.

Im not going to cover the Character class here. I believe this is something easilyresearched on your own. This is different to the char primitive data type and it is one

example of a Wrapper class.

Strings can be dealt with using three classes, which you need to get familiar with in your

APIs. (See sidebar).

The important thing to note here is that this is the first time we have encountered a Classthat will do our dirty work for us. Other languages like ABAP and C use strings as data

Alistair C. Rooney 2002 All rights reservedABAP is the registered trademark of SAP AG.Java is the registered trademark of Sun Microsystems Inc.

The Java API Documentation

One of the nicest things about Java is that you can easily see the classes you haveavailable to you and the methods you can use in these classes. Download and unzip thedocumentation for your JDK release.

The documentation is in HTML format and it has a list of all the classes down the left handside and the class detail, including the methods on the right hand side.

The HTML file you need to look at is index.htm in the API directory. Get familiar with this!Youll be using it a lot.


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types. The bonus with Java is that we can use some very powerful methods to work with

strings.

Now heres a catch.Strings are immutable in Java. This means that you cannot change a

String. Before you throw your hands up in horror let me explain that a little.

Ive had students who say to me You lied to me this works fine and they show me the

following code:

String myString = Howdy Podner;myString = Hey yall;

After they have finished swearing at me, I tell them that they have not changedthe stringmyString, they have replacedit entirely, which is a very different thing.

Declaring a String

Formally our declaration should look like this:

String myString = new String(Howdy);

This is called instantiation in Java. More on this in later lessons.

However, and this is a rather big however, we can declare a string in a shorthand way in

Java. We do not have this privilege with other classes in Java. (OK, there are one or two

but Im not telling).

The shorthand declaration is the one weve seen above:

String myString = Howdy Podner;

Concatenating Strings

Joining or concatenating strings in Java is very easy. Although we may not overload

operators in Java, Sun have done one for us. The + sign is used to concatenate two

Strings, a char and a String, and just about any other primitive data type.

Lets see an example:

String helloString = Hello;String worldString = World;System.out.println(helloString + worldString);

Or even

String myName = Alistair;



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System.out.println(myName+ is +41);

Which will print out:

Alistair is 41.

The String methods

There are over 50 methods in the String class. Obviously it would be a little silly to cover

all of them. Im going to cover a few basics. Youre job is to research the others in theAPI documentation. Ill introduce each with some code fragments.

The charAt method.

String myString = Welcome;

char firstChar = myString.charAt(3);

Important Note: The charAt method requires an integer. This integer is the index of thestring. A strings index always starts at 0.

As you may have guessed, the charAt method returns a character. We have asked it toreturn a character at position 3 which will be c.

Since this is the first time we have seen a class call, lets direct our attention to the dotoperator. This is very similar to => in ABAP.

We have the object (reference) myString. We know we want to use the charAt method.So we join the two using a dot. I.e. Object.method(). (Dont forget the brackets).

The substring method.

String catMat = The Cat sat on the Mat;String catString = catMat.substring(4,7);

The catch with this method is that the start index is normal but the end index requires that

you add 1. So to return a string Cat we start at 4 and end at 6+1 or 7.

As with charAt there is more than one variant of this method depending on theparameters you want to use.

The equals method.

String first = First String;String second = Next one;



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if (first.equals(second)){

System.out.println(they are equal!);}

Hold on. Hold on. Why cant I just use first = = second? Well Im glad you asked!

Well the answer is, you are not comparing two Strings, you are in fact comparing two

object references, which will probably not be equal even if the String values are.

So if you compare any two object references you should use the equals method.

The length method.

The length method is very useful to (surprise!) determine the length of the String.

String longOne = This String is longer than the others;If (longOne.length() > 20){

do some code here}

You may want to use the length in a loop (we will cover these soon!) but be aware that

method calls within loops have a performance overhead. Rather extract the length outside

of the loop.

Thats all on string methods. I strongly recommend you read through the APIs to see

what methods you have available to you.

Lets look at the StringBuffer class next.

StringBuffer

When using strings the one drawback is that they are immutable i.e. they cannot be

altered in situ. The solution to this problem (on the rare occasion it becomes a problem) is

to use the StringBuffer class instead of a string or as well as a string.

When using StringBuffer you are free to change the string.

Once again I will be looking at a small selection of the available methods and I encourage

you to read through the APIs.



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The first thing I need to mention is that if there is a String method to do something, then

there is probably a StringBuffer method as well. A good example of this is the charAt()

method.

Let us concentrate on the methods that are peculiar to the StringBuffer class.

The append method.

The StringBuffer class must be fully instantiated.

StringBuffer sb = new StringBuffer(my jolly string);

sb = sb.append(s);

The character s will be appended to the end of the string and the instance sb will now

contain the string my jolly strings.

Bear in mind that you can append floats, doubles, integers and other data types. Have a

quick peep at the append methods in the APIs.

Of course java gives you the ability to insert a data type at a specified position instead of

at the end. To do this use :

The insert method.

Using the same example above and assuming the instance sb has been constructed, wecould say:

sb = sb.insert(1, e);

The first parameter is the offset (from 0), which must be an integer and the second is thecharacter (in this case) that I want to insert. The end result of this operation will have sb

containing the string me jolly string.

The other methods in StringBuffer are pretty much self explanatory. The reverse methoddoes just that. It reverses a String like abcdefg and changes the buffer to gfedcba.

The toString method is handy, but since the String class takes a StringBuffer as an

argument in the constructer we could just say:

String newString = sb;

Finally I want to mention the StringTokenizer class because of its useful characteristics.



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StringTokenizer

I dont want you to spend longer than a few minutes on this class. It is useful and bearsmentioning, but it is not used that often.

StringTokenizer will allow the program to run in a loop examining the contents of astring and breaking it up into separate strings based on the delimiterthat was specified.

This is great forparsingXML files and the like.

You must loop through your string (more on loops in future lessons) and identify each

substring between the delimiters.

Heres a quick example:

String textExample = #First String#Second String#Third one;

StringTokenizer st = new StringTokenizer(textExample,#);

while (st.hasMoreTokens()){

String theToken = st.nextToken();System.out.println(theToken);

}

Note the usage of the hasMoreTokens() method to check whether we have any left andthe nextToken() method to return the next Token (strangely enough!) in the String.

Our little code snippet here will return the following:

First String

Second String

Third one

Thats all for this Lesson folks!

Come back next week where we talk about Control Flow.


java for abap programmers 7

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