java swing scroll can not work

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modify

error code

    JScrollPane scroll = new JScrollPane(jt);
    scroll.setViewportView(jt);            
    scroll.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);  
    scroll.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
    jt.setPreferredSize(new Dimension(200,100));   

Should

jt.setPreferredSize(new Dimension(200,100));

Change

scroll.setPreferredSize(new Dimension(200,100));