Java Beans - Retrieve getter-method/property by name

Author

Retrieve getter-method/property by name

Christian

2004-12-09, 8:57 am

Dear all,

my problem is that I have to sort/group loads of beans in various
ways.
I'll do this by using java.util.Comparators.
I'll extract the data to be compared from my beans via getter methods.
I would like to use a generic approach for the comparator, i.e. you
simply pass it a getter method and it will compare based on the
natural ordering of the object returned by the getter.

Sample
<code>
public class SomeComp implements Comparator, Serializable {

Method getter;
public SomeComp(Method g){
this.getter = g;
}
/* (non-Javadoc)
* @see java.util.Comparator#compare(java.lang.Object,
java.lang.Object)
*/
public int compare(Object arg0, Object arg1) {
Comparable c0 = this.getter.invoke(arg0, null);
Comparable c1 = this.getter.invoke(arg1, null);
return c0.compareTo(c1);
}
}
</code>

However, I still have the problem of getting the correct getter method
from the bean. I thought the java.beans package would do this for me,
like
<code>Method getGetter(String propname)<code>
but could not find an easy way.

Any ideas??

Thx
Christian

Robert Klemme

2004-12-09, 8:57 am

"Christian" <christian.oeckinghaus@vodafone.com> schrieb im Newsbeitrag
news:86dacc03.0412090306.1f4f4050@posting.google.com...
> Dear all,
>
> my problem is that I have to sort/group loads of beans in various
> ways.
> I'll do this by using java.util.Comparators.
> I'll extract the data to be compared from my beans via getter methods.
> I would like to use a generic approach for the comparator, i.e. you
> simply pass it a getter method and it will compare based on the
> natural ordering of the object returned by the getter.
>
> Sample
> <code>
> public class SomeComp implements Comparator, Serializable {
>
> Method getter;
> public SomeComp(Method g){
> this.getter = g;
> }
> /* (non-Javadoc)
> * @see java.util.Comparator#compare(java.lang.Object,
> java.lang.Object)
> */
> public int compare(Object arg0, Object arg1) {
> Comparable c0 = this.getter.invoke(arg0, null);
> Comparable c1 = this.getter.invoke(arg1, null);
> return c0.compareTo(c1);
> }
> }
> </code>
>
> However, I still have the problem of getting the correct getter method
> from the bean. I thought the java.beans package would do this for me,
> like
> <code>Method getGetter(String propname)<code>
> but could not find an easy way.
>
> Any ideas??

Use these methods in order:
http://java.sun.com/j2se/1.4.2/docs...trospector.html
http://java.sun.com/j2se/1.4.2/docs...ertyDescriptors()
http://java.sun.com/j2se/1.4.2/docs...l#getReadMethod()

Kind regards

robert

Chris Riesbeck

2004-12-10, 8:57 pm

In article <86dacc03.0412090306.1f4f4050@posting.google.com>,
christian.oeckinghaus@vodafone.com (Christian) wrote:

> Dear all,
>
> my problem is that I have to sort/group loads of beans in various
> ways.
> I'll do this by using java.util.Comparators.
> I'll extract the data to be compared from my beans via getter methods.
> I would like to use a generic approach for the comparator, i.e. you
> simply pass it a getter method and it will compare based on the
> natural ordering of the object returned by the getter.

I'd recommend a simpler design. No need for Method or reflection.

> public class SomeComp implements Comparator, Serializable {

add "abstract"

>
> Method getter;
> public SomeComp(Method g){
> this.getter = g;
> }

Drop these lines. Add this line:

abstract public Comparable get(Object obj);

> public int compare(Object arg0, Object arg1) {
> Comparable c0 = this.getter.invoke(arg0, null);
> Comparable c1 = this.getter.invoke(arg1, null);
> return c0.compareTo(c1);

Replace with

public int compare(Object obj1, Object obj2) {
return get(obj1).compareTo(get(obj2));
}

> }
> }

Now you can sort a list of beans using any getter you want,
e.g., to sort a list of PersonName's by last name:

Collections.sort(names, new SomeComp() {
public Comparable get(Object obj) {
return ((PersonName) obj).getLastName();
}
});

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