How to define a renderer for your layer

static void defineUniqueValueRenderer(IGeoFeatureLayer geoFeatureLayer, String
    fieldName)throws Exception{

    IRandomColorRamp colorRamp = new RandomColorRamp();
    //Make the color ramp for the symbols in the renderer.
    colorRamp.setMinSaturation(20);
    colorRamp.setMaxSaturation(40);
    colorRamp.setMinValue(85);
    colorRamp.setMaxValue(100);
    colorRamp.setStartHue(76);
    colorRamp.setEndHue(188);
    colorRamp.setUseSeed(true);
    colorRamp.setSeed(43);

    //Make the renderer.
    IUniqueValueRenderer uniqueValueRenderer = new UniqueValueRenderer();

    ISimpleFillSymbol simpleFillSymbol = new SimpleFillSymbol();
    simpleFillSymbol.setStyle(esriSimpleFillStyle.esriSFSSolid);
    simpleFillSymbol.getOutline().setWidth(0.4);

    //These properties should be set prior to adding values.
    uniqueValueRenderer.setFieldCount(1);
    uniqueValueRenderer.setField(0, fieldName);
    uniqueValueRenderer.setDefaultSymbol((ISymbol)simpleFillSymbol);
    uniqueValueRenderer.setUseDefaultSymbol(true);

    IDisplayTable displayTable = (IDisplayTable)geoFeatureLayer;
    IFeatureCursor featureCursor = (IFeatureCursor)displayTable.searchDisplayTable
        (null, false);
    IFeature feature = featureCursor.nextFeature();


    boolean valueFound;
    int fieldIndex;

    IFields fields = featureCursor.getFields();
    fieldIndex = fields.findField(fieldName);
    while (feature != null){
        ISimpleFillSymbol classSymbol = new SimpleFillSymbol();
        classSymbol.setStyle(esriSimpleFillStyle.esriSFSSolid);
        classSymbol.getOutline().setWidth(0.4);

        String classValue = (String)feature.getValue(fieldIndex);

        //Test to see if this value was added
        //to the renderer. If not, add it.
        valueFound = false;
        for (int i = 0; i <= uniqueValueRenderer.getValueCount() - 1; i++){
            if (uniqueValueRenderer.getValue(i).equals(classValue)){
                valueFound = true;
                break; //Exit the loop if the value was found.
            }
        }
        //If the value was not found, it is new and it will be added.
        if (valueFound == false){
            uniqueValueRenderer.addValue(classValue, fieldName, (ISymbol)classSymbol)
                ;
            uniqueValueRenderer.setLabel(classValue, classValue);
            uniqueValueRenderer.setSymbol(classValue, (ISymbol)classSymbol);
        }

        feature = featureCursor.nextFeature();
    }
    //Since the number of unique values is known, 
    //the color ramp can be sized and the colors assigned.
    colorRamp.setSize(uniqueValueRenderer.getValueCount());
    boolean[] ok = new boolean[1];
    colorRamp.createRamp(ok);

    IEnumColors enumColors = colorRamp.getColors();
    enumColors.reset();
    for (int j = 0; j <= uniqueValueRenderer.getValueCount() - 1; j++){
        String xv = uniqueValueRenderer.getValue(j);
        if (!xv.equals("")){
            ISimpleFillSymbol pSimpleFillColor = (ISimpleFillSymbol)
                uniqueValueRenderer.getSymbol(xv);
            pSimpleFillColor.setColor(enumColors.next());
            uniqueValueRenderer.setSymbol(xv, (ISymbol)pSimpleFillColor);
        }
    }

    //'** If you didn't use a predefined color ramp
    //'** in a style, use "Custom" here. Otherwise,
    //'** use the name of the color ramp you selected.
    uniqueValueRenderer.setColorScheme("Custom");
    ITable pTable = (ITable)displayTable;
    boolean isString = pTable.getFields().getField(fieldIndex).getType() ==
        esriFieldType.esriFieldTypeString;
    uniqueValueRenderer.setFieldType(0, isString);
    geoFeatureLayer.setRendererByRef((IFeatureRenderer)uniqueValueRenderer);

    //This makes the layer properties symbology tab
    //show the correct interface.
    IUID uid = new UID();
    uid.setValue("{683C994E-A17B-11D1-8816-080009EC732A}");
    geoFeatureLayer.setRendererPropertyPageClassID((UID)uid);

}