The attached test throws the exception:

java.lang.StringIndexOutOfBoundsException: String index out of range: 6
        at java.lang.String.substring(String.java:1777)
        at java.awt.TextComponent.getSelectedText(TextComponent.java:248)
        at Test.init(Test.java:15)
        at sun.applet.AppletPanel.run(AppletPanel.java:405)
        at java.lang.Thread.run(Thread.java:595)

---------------------SOURCE CODE---------------------

import java.awt.*;

public class Test
{
    public static final void main(String args[]) {
        TextField tf = new TextField("text field");
        tf.setSelectionStart(6);
        tf.setText("text");
String str = tf.getSelectedText();
    }
}

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it's reproducible on all platforms using JDK7 b07.