《Thinking in Java》中文版 第四章 初始化与清理

public class overloading t static Test monitor =new Test ( public static void main(String[] args)i for (int t info oi / Over loaded constructor new Tree( monitor. expect (new String[] t Tree is o feet tall" overloaded method: Tree is o feet tall "Creating new Tree that is 1 feet tall", "Tree is 1 feet tall", overloaded method Tree is 1 feet tall# "Creating new Tree that is 2 feet tall", Tree feet tall overloaded method: Tree is 2 feet tall "Creating new Tree that is 3 feet tall" 3 feet tall" overloaded method: Tree is 3 feet tall" Creating new Tree that is 4 feet tall "Tree is 4 feet tall", overloaded method: Tree is 4 feet tall", Planting a seedling }///: 创建Tree对象的时候,既可以不用参数,也可以用一个表示高度的int 当参数。前者表示它还是一棵树苗,后者表示它已经成材了。为了能提供 这种功能,程序得有一个默认的构造函数,以及一个需要拿现有高度作参 数的构造函数 或许你还想多要几种调用info()的方法。比如,想多打印些东西的时 候,你可以用info( String),什么都不想说的时候,可以用info() 同一种概念要用两个名字,那肯定会很怪。所幸的是,有了方法重载,你 就可以让这两者使用同一个名字。 区分经重载的方法 如果方法的名字相同,那么Java又该如何判断你要用哪个方法的呢?有 个很简单的规则:每一个经过重载的方法都必须有唯一的参数类型的列 表 只要稍微想一下,你就会发现这是最合情理的办法。如果不是用参数的类 型,程序员又怎么来区分两个有着相同名字的方法呢? 第6页共42页

shhgs@wgqqh.com ￾ 6 ✁ ✂ 42 ✁ public class Overloading { static Test monitor = new Test(); public static void main(String[] args) { for(int i = 0; i < 5; i++) { Tree t = new Tree(i); t.info(); t.info("overloaded method"); } // Overloaded constructor: new Tree(); monitor.expect(new String[] { "Creating new Tree that is 0 feet tall", "Tree is 0 feet tall", "overloaded method: Tree is 0 feet tall", "Creating new Tree that is 1 feet tall", "Tree is 1 feet tall", "overloaded method: Tree is 1 feet tall", "Creating new Tree that is 2 feet tall", "Tree is 2 feet tall", "overloaded method: Tree is 2 feet tall", "Creating new Tree that is 3 feet tall", "Tree is 3 feet tall", "overloaded method: Tree is 3 feet tall", "Creating new Tree that is 4 feet tall", "Tree is 4 feet tall", "overloaded method: Tree is 4 feet tall", "Planting a seedling" }); } } ///:~ LM Tree ¥K(MsÝEXˆI[ÝEˆ3ׇ^‰z( int Iš‡^A3׈Øš‡^AôõŽÙ,™,;de †Ú;â'‰m3×ÒÙ(GÚHIEh3×î%†ím‰z1 I(GÚHI ²Û¡|V%i†Nˆ info( )(µ8Ä|V]ÈÌÜÝ( M¡ÝEˆ info(String)ZÆX|$(MÝEˆ info( ) ‡3†ÖJ%ˆe×Ñ\YÞOºŠßD_(m,µ«¡ ÝEÇ ešçˆ‡3×Ñ\
 Äŵ(Ñ\>‡YZ Java üwÄÀÁ¡%ˆr×µ(ÎÊ m 3׊uv(ày[3×õ^«(µÆpqmË3(I(á ‡ ó%â¬|3k¡ºìí bw5¢(ÍÄÅXˆI( â')üÕZQ•e×m.>‡Ñ\(µÎÊ

即使是参数顺序的颠倒也足以将两个方法区分开来:(不过,通常情况下 别用这种办法,因为这会产生难以维护的代码。) // c04: Overloadingorder Overloading based on the order of the arguments import com. bruceeckel. simpletest *i public class overloadingorder t static Test monitor new Test ( static void print (string s, int i)t System. out. println("string: +s Int static void print (int i, String s) t System. out. println ("int: " i + String public static void main(String[ args)t print ("String first", 1l)i print (99,"Int first")i monitor. expect (new string[] t wint: 99, String: Int firsti r "String: String first, int: 1l' }///:~ 两个 print()有着相同的参数,但是顺序不同,而这是它们唯一不同的 地方。 对 primitive进行重载 primitive会自动地从较小的类型升级到较大的类型。如果重载的时候又 碰到这个问题,那就有点让人麻烦了。下面这段程序演示了向重载方法传 primitive的时候,会发生什么事: //: c04: Primitiveoverloading java Promotion of primitives and overloading mport com. bruceeckel. simpletest. public class Primitiveoverloading static Test monitor new Test ( void fl(char x) System. out. printin(" fl(char)")i void fl(byte x)i System. out. println("fl(byte)")K t System. out. println("fl(short)")i j void fl(int x) System. out. println("fl(int)")i] ("f1(1ong)"); void fl(float x t System. out. println("fl(float)")i j id fl(double x) roid f2(byte x)i System. out. println("f2(byte)")i K 第7页共42页

shhgs@wgqqh.com ￾ 7 ✁ ✂ 42 ✁ ºçIz'(ãä[­Ee×µ•Qy(X^·5k ˆ †Íæ™ ºå0UEæç(Œ€) //: c04:OverloadingOrder.java // Overloading based on the order of the arguments. import com.bruceeckel.simpletest.*; public class OverloadingOrder { static Test monitor = new Test(); static void print(String s, int i) { System.out.println("String: " + s + ", int: " + i); } static void print(int i, String s) { System.out.println("int: " + i + ", String: " + s); } public static void main(String[] args) { print("String first", 11); print(99, "Int first"); monitor.expect(new String[] { "String: String first, int: 11", "int: 99, String: Int first" }); } } ///:~ e× print( )m.>‡(Iz'X‡à AcË3X‡( rµ prim i tive  p ri m iti ve ºý rè(éêgèÂ(ÄÅ«(Mü .g ×!"YmCǪ©ª,k¶ „â'Õ^,뫵ì p ri m iti ve (Mºì0Zy //: c04:PrimitiveOverloading.java // Promotion of primitives and overloading. import com.bruceeckel.simpletest.*; public class PrimitiveOverloading { static Test monitor = new Test(); void f1(char x) { System.out.println("f1(char)"); } void f1(byte x) { System.out.println("f1(byte)"); } void f1(short x) { System.out.println("f1(short)"); } void f1(int x) { System.out.println("f1(int)"); } void f1(long x) { System.out.println("f1(long)"); } void f1(float x) { System.out.println("f1(float)"); } void f1(double x) { System.out.println("f1(double)"); } void f2(byte x) { System.out.println("f2(byte)"); }

oid f2(short x) t System. out. println("f2(short)")i) void f2(int x) System. out. println("f2(int)")i 1 void f2 (long x)[ System. out. println("f2(long)")iJ void f2(float x) t System. out. println(" f2(float)")i F void f2(double x) t System. out. println("f2(double)")i) id f3(short x) t System. out. println("f3(short)")i d f3(int x) Syst tln ("f3(int)") void f3(long x)[ System. out. println ("f3(long)")i t System. out. println("f3(float)")i void f3(double x) t System. out. println("f3(double)")i) void f4(int x)( System. out. println("f4(int)")i void f4(long x)i System. out. println("f4(long)")i F void f4(float x t System. out. println(f4(float)")i F t System. out. println(" f4(double)")i) void f5 (long x) System. out. println("f5(long)")i F t System. out. println(" f5(float)")i F oid f5(double x) t Syst id f6(doubl I System. out. println("f6(double)")i) void f7(double x) t System. out. println("f7(double)")i) void testConstVal() i Sy: ting (5);f2(5);f3(5);f4(5);f5(5);f6(5 void testchar()( System. out. println("char argument: " 1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); id testByte() ut.pi f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x) () System. out. println("short argument: ")i f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); void testInt ( 0 System. out. println("int argument: ")i 第8页共42页

shhgs@wgqqh.com ￾ 8 ✁ ✂ 42 ✁ void f2(short x) { System.out.println("f2(short)"); } void f2(int x) { System.out.println("f2(int)"); } void f2(long x) { System.out.println("f2(long)"); } void f2(float x) { System.out.println("f2(float)"); } void f2(double x) { System.out.println("f2(double)"); } void f3(short x) { System.out.println("f3(short)"); } void f3(int x) { System.out.println("f3(int)"); } void f3(long x) { System.out.println("f3(long)"); } void f3(float x) { System.out.println("f3(float)"); } void f3(double x) { System.out.println("f3(double)"); } void f4(int x) { System.out.println("f4(int)"); } void f4(long x) { System.out.println("f4(long)"); } void f4(float x) { System.out.println("f4(float)"); } void f4(double x) { System.out.println("f4(double)"); } void f5(long x) { System.out.println("f5(long)"); } void f5(float x) { System.out.println("f5(float)"); } void f5(double x) { System.out.println("f5(double)"); } void f6(float x) { System.out.println("f6(float)"); } void f6(double x) { System.out.println("f6(double)"); } void f7(double x) { System.out.println("f7(double)"); } void testConstVal() { System.out.println("Testing with 5"); f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5); } void testChar() { char x = 'x'; System.out.println("char argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testByte() { byte x = 0; System.out.println("byte argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testShort() { short x = 0; System.out.println("short argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testInt() { int x = 0; System.out.println("int argument:");

f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); void testlong() i System. out. println("long argument: " )i f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); void testFloato( float x=0 System. out. println("float argument: f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); void testDoubleoi double 0 System. out. println("double argument: " f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); publ ic void main(St Primitiveoverloading p new PrimitiveOverloading () p. testConstval( p. testchar()i ttt p ashort( p testInt () ttt sss Long () tFloat( p.te monitor. expect (new String[] i "Testing with 5", 234 fffff nnnn "f7(double)", "char argument fl(char) "f3(int) f6(float)", 7(double) byte argument: 123 (byte)", eer "f5(1ong)" ffs 7(double) nfl(short)nnt: " "f2(short) f3(short) "f4(int)# "f6( "f7 1fd o1o 第9页共42页

shhgs@wgqqh.com ￾ 9 ✁ ✂ 42 ✁ f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testLong() { long x = 0; System.out.println("long argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testFloat() { float x = 0; System.out.println("float argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testDouble() { double x = 0; System.out.println("double argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } public static void main(String[] args) { PrimitiveOverloading p = new PrimitiveOverloading(); p.testConstVal(); p.testChar(); p.testByte(); p.testShort(); p.testInt(); p.testLong(); p.testFloat(); p.testDouble(); monitor.expect(new String[] { "Testing with 5", "f1(int)", "f2(int)", "f3(int)", "f4(int)", "f5(long)", "f6(float)", "f7(double)", "char argument:", "f1(char)", "f2(int)", "f3(int)", "f4(int)", "f5(long)", "f6(float)", "f7(double)", "byte argument:", "f1(byte)", "f2(byte)", "f3(short)", "f4(int)", "f5(long)", "f6(float)", "f7(double)", "short argument:", "f1(short)", "f2(short)", "f3(short)", "f4(int)", "f5(long)", "f6(float)", "f7(double)

f2(int) 6(f1oat)", f7(double) fl (long) "f4(1ong)", f5 (long) float argument: " fl(float)", f2(float) "f3(float)", f 4(float) "f5(float)", "f6(float)" double argument: " "fl(double)", f 4(double) 你会发现,常量5会被当作int,所以,如果重载的方法里有拿int作参 数的,它会用这个方法。在其他情况下,如果实参(译者注:真正调用方 法的那个参数)的类型比形参(译者注:方法的定义中所声明的参数)的类 型小,那么数据会先提升。char的反映会有些不同,因为如果如果找不 到匹配的char的话,它会被提升到int 如果实参比形参更大,那又会怎样呢?我们修改一下上面那段程序,让它 来告诉你答案 Demotion of primitives and ov er⊥ oading import com. bruceeckel. simpletest *i static Test monitor new Test ( void fl(char x)[ System. out. println("fl(char)")iJ t System. out. println("fl(short)")i j 第10页共42页

shhgs@wgqqh.com ￾ 10 ✁ ✂ 42 ✁ "int argument:", "f1(int)", "f2(int)", "f3(int)", "f4(int)", "f5(long)", "f6(float)", "f7(double)", "long argument:", "f1(long)", "f2(long)", "f3(long)", "f4(long)", "f5(long)", "f6(float)", "f7(double)", "float argument:", "f1(float)", "f2(float)", "f3(float)", "f4(float)", "f5(float)", "f6(float)", "f7(double)", "double argument:", "f1(double)", "f2(double)", "f3(double)", "f4(double)", "f5(double)", "f6(double)", "f7(double)" }); } } ///:~ ¡ºìí·+ 5 ºB 1 intDEÄÅ«(µËm† int 1 I(Aºˆ ×µF§G5kÄÅñ(šyíîNˆµ (Y×I)(8ï(šyµ(O¿*Dðñ(I)( YZIòº/déchar (ó¦ºmÌX‡æ™ÄÅÄÅôX gõ( char (›AºBdég int ÄÅñ8ïÂYüºÕ¦ÎÊ
cïð3kJ¶Y„â'ÇA Q#¡öty //: c04:Demotion.java // Demotion of primitives and overloading. import com.bruceeckel.simpletest.*; public class Demotion { static Test monitor = new Test(); void f1(char x) { System.out.println("f1(char)"); } void f1(byte x) { System.out.println("f1(byte)"); } void f1(short x) { System.out.println("f1(short)"); }