褰撲竴涓簲鐢ㄥ湪闇瑕佷竴涓璞＄殑鍦版柟璇曞浘浣跨敤 null 鏃舵姏鍑恒傚畠浠寘鎷細
- 璋冪敤涓涓?
null瀵硅薄鐨勫疄渚嬫柟娉曘? - 璁塊棶鎴栦慨鏀逛竴涓?
null瀵硅薄鐨勫煙銆? - 灝?
null浣滀負涓涓暟緇勶紝浣跨敤瀹冪殑闀垮害銆? - 灝?
null浣滀負涓涓暟緇勶紝璁塊棶鎴栦慨鏀瑰畠鐨勬彃鍙ｃ? - 灝?
null浣滀負涓涓?Throwable鍊兼姏鍑恒?
搴旂敤搴旇鎶涘嚭璇ョ被鐨勫疄渚嬶紝鎸囩ず鍏跺畠瀵?null 瀵硅薄鐨勯潪娉曚嬌鐢ㄣ?BR>
鏋勯犲瓙绱㈠紩
- NullPointerException()
- 鏋勯犱竴涓病鏈夎緇嗘秷鎭殑
NullPointerException銆? - NullPointerException(String)
- 鏋勯犱竴涓叿鏈夋寚瀹氳緇嗘秷鎭殑
NullPointerException
鏋勯犲瓙
NullPointerExceptionpublic NullPointerException()
- 鏋勯犱竴涓病鏈夎緇嗘秷鎭殑
NullPointerException銆?
public NullPointerException(String s)
- 鏋勯犱竴涓叿鏈夋寚瀹氳緇嗘秷鎭殑
NullPointerException銆?- 鍙傛暟錛?/B>
- s - 璇︾粏娑堟伅銆?
- 鏋勯犱竴涓病鏈夎緇嗘秷鎭殑
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1. Garbage collection is a mechanism for reclaiming memory from objects that are no longer in use, and making the memory available for new objects.
2. An object being no longer in use means that it can鈥檛 be referenced by any 鈥榓ctive鈥?part of the program.
3. Garbage collection runs in a low priority thread. It may kick in when memory is too low. No guarantee.
4. It鈥檚 not possible to force garbage collection. Invoking System.gc may start garbage collection process.
5. There are no guarantees that the objects no longer in use will be garbage collected and their finalizers executed at all. gc might not even be run if the program execution does not warrant it. Thus any memory allocated during program execution might remain allocated after program termination, unless reclaimed by the OS or by other means.
6. There are also no guarantees on the order in which the objects will be garbage collected or on the order in which the finalizers are called.
7. Circular references do not prevent objects from being garbage collected.
8. We can set the reference variables to null, hinting the gc to garbage collect the objects referred by the variables. Even if we do that, the object may not be gc-ed if it鈥檚 attached to a listener. (Typical in case of AWT components) Remember to remove the listener first.
9. All objects have a finalize method. It is inherited from the Object class.
10. finalize method is used to release system resources other than memory. (such as file handles and network connections) The order in which finalize methods are called may not reflect the order in which objects are created. Don鈥檛 rely on it. This is the signature of the finalize method.
protected void finalize() throws Throwable { }
In the descendents this method can be protected or public. Descendents can restrict the exception list that can be thrown by this method.
11. finalize is called only once for an object. If any exception is thrown in finalize, the object is still eligible for garbage collection (at the discretion of gc)
12. gc keeps track of unreachable objects and garbage-collects them, but an unreachable object can become reachable again by letting know other objects of its existence from its finalize method (when called by gc). This 鈥榬esurrection鈥?can be done only once, since finalize is called only one for an object.
13. finalize can be called explicitly, but it does not garbage collect the object.
14. finalize can be overloaded, but only the method with original finalize signature will be called by gc.
15. finalize is not implicitly chained. A finalize method in sub-class should call finalize in super class explicitly as its last action for proper functioning. But compiler doesn鈥檛 enforce this check.
16. System.runFinalization can be used to run the finalizers (which have not been executed before) for the objects eligible for garbage collection.
17. Local variables in methods go out of scope when the method exits. At this point the methods are eligible for garbage collection. Each time the method comes into scope the local variables are re-created.
18. Java uses a "mark sweep garbage collection algorithm, which traverses all the object references, marking any objects that are referred to and then garbage collecting any objects that are unmarked.
19. Java allows you to add a finalize() method to any class. The finalize() method will be called before the garbage collector sweeps away the object. In practice, do not rely on the finalize method for recycling any resources that are in short supply - you simply cannot know when this method will be called.
In the exam point of view :
飩鳳燳ou must be able to identify when an object is available for gc - you have either set it to null or you
have "redirected" the variable that was originally referring to it, so that it now refers to a different
object.
飩鳳爄f you have a reference to an object say, A and then you pass A as an argument to some constructor -
new obj(A); - then even if you null your reference - A=null; - you can't say that A is available for
gc. So just follow the references and when they drop to zero you know its eligible/available for gc,
not that it will happen.
I can not full understand these statements which are above.
eg,
1. obj = new Jo();
2. obj.doSomething();
3. obj = new Jo(); //Same as obj=null;
4. obj.doSomething();
Object a = new Object();
Object a=null; //Now the object created in 1st line is available for gc
Object a=new Object();
a = new Object(); //same.
// Now original object created in line 1 is available for gc and a new
object is now out there referenced by "a".
Aclass a = new Aclass(); // Object 1
Aclass b= new Aclass(); // Object 2
Aclass c = new Aclass(); // Object 3
a=b; // now we have no valid object reference to object "a" and it will be
// garbage collected sometime after this statement. But when?......
a=c;
c=null; // no garbage collection will be eligible since
// "a" still refers to Object 3
a=null; // now object "c" is eligible for gc since it always had a valid reference.
// Should "b" go out of scope; then we would possibly have eligibility for gc.
// there might still be other references to object "b" preventing the collection.
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1. A command to change the text.
2. A color (hex) code.
<LINK="######"> -- This denotes the color of the links on your page.
<ALINK="######"> -- This denotes the color the link will flash when clicked upon.
<VLINK="######"> -- This denotes the colors of the links after they have been visited.
These commands come right after the <TITLE> commands. Again, in that position they affect everything on the page. Also... place them all together inside the same command along with any background commands. Something like this:
< BODY BGCOLOR="######" TEXT="######" LINK="######" VLINK="######">
Please note: When you write these codes, you can write them with a # sign in front of the hex code or not. It used to be that that was required, but not any more. I still use it just because I started that way. You may want to just go with the six-digit code. Also make sure to place a space between each command and be sure to enclose it in quotation marks, like so:
<VLINK="#FFFFFF">
闂錛?BR>銆銆涓婇潰鐨勯偅鍑犱釜涓滆タ榪樻病鏈夋悶鎳傦紟涓嶇煡閬撲粈涔堢敤錛?BR>
Changing Specific Word ColorBut I only want to change one word's color!
You'll use a color (hex) code to do the trick. Follow this formula:
It's a pain in the you-know-where, but it gets the job done. It works with all H commands and text size commands. Basically, if it's text, it will work.
]]>
"Aqua" and "Cyan" produce the same color: 00FFFF
"Fuchsia" and "Magenta" produce the same color: FF00FF
| Aliceblue F0F8FF |
Antiquewhite FAEBD7 |
Aqua 00FFFF |
| Aquamarine 7FFFD4 |
Azure F0FFFF |
Beige F5F5DC |
| Bisque FFE4C4 |
Black 000000 |
Blanchedalmond FFEBCD |
| Blue 0000FF |
Blueviolet 8A2BE2 |
Brown A52A2A |
| Burlywood DEB887 |
Cadetblue 5F9EA0 |
Chartreuse 7FFF00 |
| Chocolate D2691E |
Coral FF7F50 |
Cornflowerblue 6495ED |
| Cornsilk FFF8DC |
Crimson DC143C |
Cyan 00FFFF |
| Darkblue 00008B |
Darkcyan 008B8B |
Darkgoldenrod B8860B |
| Darkgray A9A9A9 |
Darkgreen 006400 |
Darkkhaki BDB76B |
| Darkmagenta 8B008B |
Darkolivegreen 556B2F |
Darkorange FF8C00 |
| Darkorchid 9932CC |
Darkred 8B0000 |
Darksalmon E9967A |
| Darkseagreen 8FBC8F |
Darkslateblue 483D8B |
Darkslategray 2F4F4F |
| Darkturquoise 00CED1 |
Darkviolet 9400D3 |
deeppink FF1493 |
| Deepskyblue 00BFFF |
Dimgray 696969 |
Dodgerblue 1E90FF |
| Firebrick B22222 |
Floralwhite FFFAF0 |
Forestgreen 228B22 |
| Fuchsia FF00FF |
Gainsboro DCDCDC |
Ghostwhite F8F8FF |
| Gold FFD700 |
Goldenrod DAA520 |
Gray 808080 |
| Green 008000 |
Greenyellow ADFF2F |
Honeydew F0FFF0 |
| Hotpink FF69B4 |
Indianred CD5C5C |
Indigo 4B0082 |
| Ivory FFFFF0 |
Khaki F0E68C |
Lavender E6E6FA |
| Lavenderblush FFF0F5 |
Lawngreen 7CFC00 |
Lemonchiffon FFFACD |
| Lightblue ADD8E6 |
Lightcoral F08080 |
Lightcyan E0FFFF |
| Lightgoldenrodyellow FAFAD2 |
Lightgreen 90EE90 |
Lightgrey D3D3D3 |
| Lightpink FFB6C1 |
Lightsalmon FFA07A |
Lightseagreen 20B2AA |
| Lightskyblue 87CEFA |
Lightslategray 778899 |
Lightsteelblue B0C4DE |
| Lightyellow FFFFE0 |
Lime 00FF00 |
Limegreen 32CD32 |
| Linen FAF0E6 |
Magenta FF00FF |
Maroon 800000 |
| Mediumauqamarine 66CDAA |
Mediumblue 0000CD |
Mediumorchid BA55D3 |
| Mediumpurple 9370D8 |
Mediumseagreen 3CB371 |
Mediumslateblue 7B68EE |
| Mediumspringgreen 00FA9A |
Mediumturquoise 48D1CC |
Mediumvioletred C71585 |
| Midnightblue 191970 |
Mintcream F5FFFA |
Mistyrose FFE4E1 |
| Moccasin FFE4B5 |
Navajowhite FFDEAD |
Navy 000080 |
| Oldlace FDF5E6 |
Olive 808000 |
Olivedrab 688E23 |
| Orange FFA500 |
Orangered FF4500 |
Orchid DA70D6 |
| Palegoldenrod EEE8AA |
Palegreen 98FB98 |
Paleturquoise AFEEEE |
| Palevioletred D87093 |
Papayawhip FFEFD5 |
Peachpuff FFDAB9 |
| Peru CD853F |
Pink FFC0CB |
Plum DDA0DD |
| Powderblue B0E0E6 |
Purple 800080 |
Red FF0000 |
| Rosybrown BC8F8F |
Royalblue 4169E1 |
Saddlebrown 8B4513 |
| Salmon FA8072 |
Sandybrown F4A460 |
Seagreen 2E8B57 |
| Seashell FFF5EE |
Sienna A0522D |
Silver C0C0C0 |
| Skyblue 87CEEB |
Slateblue 6A5ACD |
Slategray 708090 |
| Snow FFFAFA |
Springgreen 00FF7F |
Steelblue 4682B4 |
| Tan D2B48C |
Teal 008080 |
Thistle D8BFD8 |
| Tomato FF6347 |
Turquoise 40E0D0 |
Violet EE82EE |
| Wheat F5DEB3 |
White FFFFFF |
Whitesmoke F5F5F5 |
]]>
public boolean isFile()
- 嫻嬭瘯褰撳墠
File瀵硅薄琛ㄧず鐨勬枃浠舵槸鍚︽槸涓涓滄櫘閫氣濇枃浠躲?濡傛灉涓涓枃浠朵笉鏄竴涓礬寰勪笖婊¤凍鍏跺畠緋葷粺鐨勬爣鍑嗭紝鍒欏畠鏄竴涓滄櫘閫氣濇枃浠躲傜敱 Java 鐨勪竴涓簲鐢ㄧ▼搴忓垱寤虹殑浠諱綍闈炶礬寰勬枃浠惰偗瀹氭槸涓涓滄櫘閫氣濇枃浠躲?
- 榪斿洖鍊鹼細
- 濡傛灉褰撳墠瀵硅薄鎸囧畾鐨勬枃浠跺瓨鍦ㄤ笖鏄竴涓滄櫘閫氣濇枃浠跺垯涓?
true錛涘惁鍒欎負false銆? - 鎶涘嚭錛?/B> SecurityException
- 濡傛灉鏈変竴涓畨鍏ㄧ鐞嗗櫒錛屽垯鐢ㄥ綋鍓?
File鐨勮礬寰勫悕璋冪敤checkRead鏂規硶錛屾煡鐪嬫槸鍚﹀厑璁告搴旂敤紼嬪簭璇昏鏂囦歡銆?- 鍙傝錛?/B>
- getPath, checkRead
Filepublic File(File dir, String name)
- 鍒涘緩涓涓?
File瀹炰緥錛岃〃紺烘寚瀹氳礬寰勬寚瀹氬悕縐扮殑鏂囦歡銆?濡傛灉璺緞鍙傛暟涓?
null, 鍒欑粨鏋?File瀹炰緥琛ㄧず鍦ㄥ綋鍓嶈礬寰?涓庣郴緇熸湁鍏?涓嬬殑涓涓枃浠訛紝瀹冪殑璺緞鍚嶆槸name鍙傛暟銆傚惁鍒欙紝File瀹炰緥琛ㄧず涓涓枃浠訛紝瀹冪殑璺緞鍚嶆槸璺緞鍙傛暟(dir)緇欏畾鐨勮礬寰勫悕錛屽悗璺熷垎闅旂鍜?name鍙傛暟銆?- 鍙傛暟錛?
- dir - 璺緞銆?
- name - 鏂囦歡璺緞鍚嶃?
- 鍙傝錛?
-
getPath, separator
涓嬮潰鏄繖涓▼搴忕殑浠ｇ爜錛?BR>import java.io.*;
import java.lang.*;
public class Example20_4
{
public static void Traverse(File dir){
System.out.println(dir.toString());
String fileName[]=dir.list();
for(int i=0;i<fileName.length;i++){
System.out.println(fileName[i]);
File dir1=new File(dir,fileName[i]);
//String str=dir1.getPath();
//File dir2=new File(str);
if(dir1.isFile())
continue;
Traverse(dir1);
}
System.out.println();
}
public static void main(String [] args){
File dir=new File(args[0]);
Traverse(dir);
}
}
- 濡傛灉鏈変竴涓畨鍏ㄧ鐞嗗櫒錛屽垯鐢ㄥ綋鍓?
]]>