java - getPreferredSize() works on JFrame; why doesn't it work on Frame? -

i hope problem confuses me.

i tested getpreferredsize() on frame , on frame. in frame, getpreferredsize() returns 0 (both width , height), while in frame, returns non-zero numbers, working.

i not sure part of coding in nolayout() method such getpreferredsize() returns 0, , ispreferredsizeset() returns false, implies preferred size not set.

why not set? can help? thanks.

below codings of simple test program.

import java.awt.*; import javax.swing.*;  public class test { public test(){     jframe frame = new jframe("absolutelayoutdemo");     frame.setdefaultcloseoperation(jframe.exit_on_close);     container p = frame.getcontentpane();     p.setlayout(null);     jbutton b1 = new jbutton("one");     p.add(b1);     dimension size = b1.getpreferredsize();     system.out.println("size width "+size.width+" height "+size.height);     jlabel label2 = new jlabel("text-only label");     p.add(label2);     size = label2.getpreferredsize();     system.out.println("label size width "+size.width+" height "+size.height);      insets insets = p.getinsets();      frame.setsize(300 + insets.left + insets.right, 125 + insets.top + insets.bottom);     frame.setvisible(true); } public static void main(string[] args) {     test t = new test();     nolayout(); }enter code here    public static void nolayout() {      frame f = new frame();     f.setlayout(null);      insets = f.getinsets();      system.out.println("its left, "+its.left+" right, "+its.right+" top "+its.top+" bottom "+its.bottom);      f.setlocation(200, 100 );     f.setsize(its.left+its.right+300,its.top+its.bottom+200);      label l1 = new label("enter integer added : ",label.right);     textfield t1 = new textfield("0",10);      f.add(l1);     f.add(t1);       system.out.println("is l1 preferred size set "+l1.ispreferredsizeset());     dimension sz = l1.getpreferredsize();     l1.setbounds(its.left + 10, its.top + 10, sz.width, sz.height);     system.out.println("label 1 width "+sz.width+" height " +sz.height);      sz = t1.getpreferredsize();     t1.setbounds(its.left + 100, its.top + 10, sz.width, sz.height);     system.out.println("text 1 width "+sz.width+" height " +sz.height);      f.setvisible(true);     } }  

the output on command window follows:

c:\javapr>java test size width 55 height 26 label size width 88 height 16 left, 0 right, 0 top 0 bottom 0 l1 preferred size set false label 1 width 0 height 0 text 1 width 0 height 0 enter code here 

the result of getpreferredsize() reliable, once specify layout , pack() parent window. suggested here, may want override getpreferredsize().

import java.awt.dimension; import java.awt.frame; import java.awt.gridbaglayout; import java.awt.label; import java.awt.panel; import java.awt.textfield; import java.awt.event.windowadapter; import java.awt.event.windowevent;  /** @see https://stackoverflow.com/posts/37501391/edit */ public class test {      private void display() {         frame f = new frame(){             @override             public dimension getpreferredsize() {                 return new dimension(320, 240);             }         };         f.addwindowlistener(new windowadapter() {             @override             public void windowclosing(windowevent e) {                 system.exit(0);             }         });         f.setlayout(new gridbaglayout());         panel p = new panel();         p.add(new label("test", label.right));         p.add(new textfield("test", 10));         f.add(p);         f.pack();         f.setlocationrelativeto(null);         f.setvisible(true);     }      public static void main(string[] args) {         new test().display();     } }