java语言程序设计基础篇 复习题答案_《Java语言程序设计：基础篇》课后复习题答案-第十五章...

《Java语言程序设计：基础篇》课后复习题答案

Chapter15Graphics

1.

2.Theycoordinateshouldincreaseandthexcoordinateshouldremainunchanged.TheGraphicsclass–anabstractclass—providesa

device-independentgraphicsinterfacefordisplayingfiguresandimagesonthescreenondifferent

platforms.Wheneveracomponent(e.g.,abutton,alabel,apanel)isdisplayed,theJVMautomaticallycreatesaobjectforthecomponentonthenativeplatformandpassesthisobjecttoinvokethepaintComponentmethodtodisplaythedrawings.

ThepaintComponent()methodisdefinedintheComponentclass.TheJavaruntimesysteminvokesittopaintthingsonaSwingGUIcomponent.Thismethodcannotbeinvokedbythesystemorbytheprogrammer.Thesystemautomaticallyinvokesitwhenevertheviewingareachanges.Theprogrammerinvokesitthroughinvokingtherepaint()method.TheprogrammershouldneverdirectlyinvokethepaintComponent()method.

ThepaintComponent()methodisprotected,because(1)thismethodisalwaysinvokedbytheJVM,notbyaclientprogram;(2)theclientprogramneedtooverrideitinasubclass.

Ifitischangedtopublic,itisOK,butnotnecessary,becausetheprotectedmodifierissufficient.

Itcannotbechangedtoprivate,becausethevisibilitycannotbeweakened.super.paintComponent(g)invokesthesuperclass’spaintComponentmethod.InLine12inListing12.2,itcausesthetextofthelabeltobepaintedfirst.Beforethistextisdisplayed,theJLabel’spaintComponent(g)methodactuallyinvokessuper.paintComponent(g)tocleartheviewingarea.InLine20inListing12.3,itcausestheviewingareatobecleared.

5.Yes.YoushoulddeclareacustomcanvasbysubclassingJPanelratherthan

subclassingJLabelorJButton,becauselabelsaredesignedforthepurposetodisplayalabelorandbuttonsareforshowingapushbutton.

SeetheSections12.6,12.8,and12.9.

SeetheSections12.6,12.8,and12.9.

YoucanusethesetColor(Color)tosetacolorinthegraphicscontextandusethesetFont(Font)methodtosetafontinthegraphicscontext.3.4.6.7.8.