java.io.FileNotFoundException: abc.txt (系統找不到指定的檔案。) at java.io.FileInputStream.open(Native Method) at java.io.FileInputStream.<init>(Unknown Source) at java.io.FileInputStream.<init>(Unknown Source) at ExceptionExample.someMethod(ExceptionExample.java:7) at ExceptionExample.main(ExceptionExample.java:19)
java.io.FileNotFoundException: abc.txt (系統找不到指定的檔案。)
abc.txt (系統找不到指定的檔案。)另外我們故意讓程式丟出 NullPointerException，透過上面三種方法輸出例外訊息，其範例與輸出結果如下：
String str = null;
try {
str.length();
} catch (Exception e) {
e.printStackTrace();
}
java.lang.NullPointerException at ExceptionExample.main(ExceptionExample.java:30)
java.lang.NullPointerException
null在上面的 NullPointerException 例子，toString() 與 getMessage() 幾乎沒有任何幫助，基本上，除非是自訂的例外或者可以明顯知道例外的輸出的解果發生在哪個地方，例如，我們只在某個地方開啟特殊的檔案，則我們可以考慮使用 toString() 與 getMessage() 來減少除錯訊息的輸出，否則建議還是直接 printStackTrace() 方法輸出詳細的例外訊息。
"foo".equals((String)null) // 回傳 false "foo".compareTo((String)null) == 0 // 丟出 NullPointerException
if (anObject instanceof String) {
...
}
而 null 是物件的參考常數值，它可以被任何物件所參考，但它不屬於任何物件。也就是說 null instanceof object 總是回傳 false，故在將它傳入 equals method 並不會丟出任何例外。There is also a special null type, the type of the expression null (§3.10.7, §15.8.1), which has no name. Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type. The null reference is the only possible value of an expression of null type. The null reference can always undergo a widening reference conversion to any reference type. In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.
// 列出目前所有的 JVM 屬性設定
System.getProperties().list(System.out);
// 透過 getProperty 單獨取出特定的某個屬性設定值
System.out.println("Color = " + System.getProperty("file.separator"));
// 取出我們設定的屬性值，注意屬性名稱是 case sensitive
// 如果你寫成 color 得到的會是 null
System.out.println("Color = " + System.getProperty("Color"));
// 你也可以使用下列方式，當所指定的屬性沒有鍵值時，回傳你所設定的預設值
// 下面的範例當沒有設定 Color 屬性鍵值時會回傳預設的 Pink 值
System.out.println("Color = " + System.getProperty("Color", "Pink"));
// 列出目前作業系統所有的環境變數設定
Set<Map.Entry<String, String>> envSet = System.getenv().entrySet();
for (Map.Entry<String, String> set: envSet) {
System.out.println(set.getKey() + " = " + set.getValue());
}
// 透過 System.getenv(key) 的方式單獨取出你想要的屬性設定
System.out.println("SystemDrive = " + System.getenv("SystemDrive"));
System.out.println("TEMP = " + System.getenv("TEMP"));
class Point implements Cloneable {
private int x;
private int y;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
// 2. 把 setPoint 移除，讓 Point 變成 immutable 的類別
public void setPoint(int x, int y) {
this.x = x;
this.y = y;
}
public int getX() {
return x;
}
public int getY() {
return y;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
class Circle {
private Point point;
private int radius;
public Circle(int x, int y, int r) {
point = new Point(x, y);
radius = r;
}
public Point getPoint() {
return point;
}
public Point getReadOnlyPoint() {
try {
// 3. 讓 Point 類別實作 Cloneable，然後回傳 clone 物件
return (Point)point.clone();
} catch (CloneNotSupportedException e) {
e.printStackTrace();
}
return null;
}
public Point getReadOnlyPoint2() {
//1. 使用 new 的方式，將目前的Point 資料拷貝一份
return new Point(point.getX(), point.getY());
}
@Override
public String toString() {
return String.format("(%d, %d) ,radius = %d", point.getX(), point.getY(), radius);
}
}
public class ProtectReferenceObj {
public static void main(String[] args) {
Circle circle = new Circle(1,1,2);
Point pt = circle.getPoint();
System.out.println("Before: " + circle);
pt.setPoint(10, 20);
System.out.println("After: " + circle);
System.out.println("================================");
System.out.println("Before: " + circle);
Point pt2 = circle.getReadOnlyPoint();
pt2.setPoint(11, 22);
System.out.println("After: " + circle);
System.out.println("pt2.x = " + pt2.getX() + " pt2.y = " + pt2.getY());
System.out.println("================================");
System.out.println("Before: " + circle);
Point pt3 = circle.getReadOnlyPoint2();
pt3.setPoint(55, 99);
System.out.println("After: " + circle);
System.out.println("pt3.x = " + pt3.getX() + " pt3.y = " + pt3.getY());
}
}
Before: (1, 1) ,radius = 2 After: (10, 20) ,radius = 2 ================================ Before: (10, 20) ,radius = 2 After: (10, 20) ,radius = 2 pt2.x = 11 pt2.y = 22 ================================ Before: (10, 20) ,radius = 2 After: (10, 20) ,radius = 2 pt3.x = 55 pt3.y = 99
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
public T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
有上面可知，我們有兩個方式可以使用 toarray 方法，一個是不帶任何參數 toarray 方法回傳 Object 陣列，另外一個是回傳你所傳入的型態陣列。 static void toArrayDemo() {
ArrayList<integer> list = new ArrayList<integer>(){{
add(1);
add(2);
add(3);
add(4);
}};
//"main" java.lang.NullPointerException
//Integer[] test = null;
//test = list.toArray(test);
// or Integer[] test = list.toArray(null);
// 配置的元素小於 ArrayList 所存的元素各數少時，
// toArray 會幫你重新配置足夠的陣列個數來儲存。
Integer[] test = list.toArray(new Integer[0]);
System.out.print("<1>[ ");
for (Integer val : test) {
System.out.print(val + " ");
}
System.out.println("]");
// 配置大於 ArrayList 所存的 元素個數，多餘的陣列元素都是 null
Integer[] test2 = list.toArray(new Integer[10]);
System.out.print("<2>[ ");
for (Integer val : test2) {
System.out.print(val + " ");
}
System.out.println("]");
// 配置符合目前 ArrayList 所含的元素個數
Integer[] test3 = list.toArray(new Integer[list.size()]);
System.out.print("<3>[ ");
for (Integer val : test3) {
System.out.print(val + " ");
}
System.out.println("]");
//無法將 Object 陣列轉成其他型態的陣列
//[Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;
//Integer[] test4 = (Integer[] )list.toArray();
Object [] obj = list.toArray();
System.out.print("<4>[ ");
for (Object val : obj) {
// 你可以對每一個 Object 中的元素轉型成 Integer 型態的物件
// 因為我們宣告的 ArrayList 是儲存 Integer 物件，所以每一個
// Object 物件都是參考到 Integer 物件
System.out.print( ((Integer)val).intValue() + " ");
// 下面方式與上面轉型所列印出來的結果相同，因為每一個 Object 元素
// 都是參考到 Integer 物件，而 Integer 的 toString method 就是列印其值。
//System.out.print( val + " ");
}
System.out.println("]");
}
static void arrayDemo() {
Object [] intobj = new Integer[2];
System.out.println("obj instance Integer[] = " + (intobj instanceof Integer[]));
obj[0] = new Integer(10);
//Exception in thread "main" java.lang.ArrayStoreException: java.lang.Float
//obj[1] = new Float(10.f);
Object [] obj2 = new Object[2];
System.out.println("obj2 instance Integer[] = " + (obj2 instanceof Integer[]));
obj2[0] = new Integer(10);
obj2[1] = new Float(10.f);
System.out.println("obj2[0] = " + obj2[0] + " , obj2[1] = " + obj2[1]);
}
ArrayList<integer> list = new ArrayList<integer>();
list.add(2);
list.add(2);
list.add(3);
list.add(1);
list.add(2);
System.out.println(list);
for (int i=0; i<list.size(); i++){
if(list.get(i)<3) {
list.remove(i);
}
}
System.out.println(list);
Iterator<Integer> Allelement = list.iterator();
while (Allelement.hasNext()) {
if (Allelement.next() < 3) {
Allelement.remove(); // 刪除最後 Iterator 所回傳的值，也就是目前所指到值
}
}
透過 for 迴圈反向檢查：
for (int i = list.size() - 1; i >= 0; i--) {
if(list.get(i) < 3){
list.remove(i);
}
}
CopyOnWriteArrayList <Integer> list =
new CopyOnWriteArrayList <Integer>();
list.add(2);
list.add(2);
list.add(3);
list.add(1);
list.add(2);
for (Integer val : list) {
if (val < 3) {
list.remove(val);
}
}
請注意，使用 foreach 去詢訪 ArrayList 時，你不可以在 foreach 中 執行 remove，如果執行了應該會出現 java.util.ConcurrentModificationException 的例外訊息。// InputDemo.java
import java.io.IOException;
import java.util.Scanner;
public class InputDemo {
public static void main(String[] args) throws IOException {
OtherInput otherInput = new OtherInput();
int number = -1;
Scanner scanner = new Scanner(System.in);
// 嘗試執行下列敘述會得到IO例外訊息
//System.out.println("System.in.available()" + System.in.available());
System.out.print("Input a number:");
if (scanner.hasNextInt()) {
number = scanner.nextInt();
}
scanner.close();
// 因為 scanner.nextInt() 沒有執行，所以 number 為 -1
System.out.println("You input a number " + number);
}
}
// OtherInput.java
import java.util.Scanner;
public class OtherInput {
public OtherInput() {
Scanner scanner = new Scanner(System.in);
int age = 0;
String name = null;
System.out.print("Please input your name:");
if (scanner.hasNextLine()) {
name = scanner.nextLine();
}
System.out.print("Please input your age:");
if (scanner.hasNextInt()) {
age = scanner.nextInt();
}
scanner.close();
System.out.println("Your name: " + name + " and age: " + age);
}
}
Exception in thread "main" java.io.IOException: Stream closed at java.io.BufferedInputStream.getInIfOpen(Unknown Source) at java.io.BufferedInputStream.available(Unknown Source) at InputDemo.main(InputDemo.java:10)
import java.util.Scanner;
public class InputDemo {
public static void main(String[] args) {
int number = -1;
Scanner scanner = new Scanner(System.in);
System.out.print("Input a number:");
if (scanner.hasNextInt()) {
number = scanner.nextInt();
}
scanner.close();
System.out.println("You input a number " + number);
}
}
TimeZone.getDefault().getID(); // Asia/Taipei TimeZone.getDefault().getDisplayName(); // friendly name ex: 台灣標準時間
TimeZone tx = TimeZone.getTimeZone("Indian/Cocos"); // "Indian/Cocos" 就是 TimeZone ID
TimeZone.setDefault(tx);
String [] ids = TimeZone.getAvailableIDs();
for (String timezone : ids) {
System.out.println(timezone + " " +
TimeZone.getTimeZone(timezone).getDisplayName());
}
Calendar now = Calendar.getInstance();
int hour = now.get(Calendar.HOUR_OF_DAY);
int minute = now.get(Calendar.MINUTE);
int second = now.get(Calendar.SECOND);
int month = now.get(Calendar.MONTH);
int day = now.get(Calendar.DAY_OF_MONTH);
int year = now.get(Calendar.YEAR);
System.out.printf("現在是 %d/%d/%d %02d:%02d:%02d",
year, (month+1), day, hour, minute, second);
public class BooleanDemo {
public static void main(String[] args) {
System.out.println(System.getProperties());
System.out.println("======================================");
System.out.println(System.getProperty("java.runtime.name"));
System.out.println("System.getProperty(\"true\") = " + System.getProperty("true"));
System.out.println("System.getProperty(\"TRUE\") = " + System.getProperty("TRUE"));
System.out.println("System.getProperty(\"True\") = " + System.getProperty("True"));
System.out.println("Boolean.getBoolean(\"true\") = " + Boolean.getBoolean("true"));
System.out.println("Boolean.getBoolean(\"TRUE\") = " + Boolean.getBoolean("TRUE"));
System.out.println("Boolean.valueOf(\"true\") = " + Boolean.valueOf("true"));
System.out.println("Boolean.valueOf(\"TRUE\") = " + Boolean.valueOf("TRUE"));
System.setProperty("TRUE", "TRUE");
System.setProperty("YES", "True");
System.out.println("Boolean.getBoolean(\"TRUE\") = " + Boolean.getBoolean("TRUE"));
System.out.println("Boolean.getBoolean(\"YES\") = " + Boolean.getBoolean("YES"));
System.out.println("Boolean.getBoolean(\"yes\") = " + Boolean.getBoolean("yes"));
System.clearProperty("YES");
System.out.println("Boolean.getBoolean(\"YES\") = " + Boolean.getBoolean("YES"));
}
}
輸出結果：
{java.runtime.name=Java(TM) SE Runtime Environment, sun.boot.library.path=C:\Program Files\Java\jre7\bin, java.vm.version=23.25-b01, java.vm.vendor=Oracle Corporation, java.vendor.url=http://java.oracle.com/, path.separator=;, java.vm.name=Java HotSpot(TM) Client VM, file.encoding.pkg=sun.io, user.country=TW, user.script=, sun.java.launcher=SUN_STANDARD, sun.os.patch.level=Service Pack 3, java.vm.specification.name=Java Virtual Machine Specification, user.dir=C:\Documents and Settings\Josh\JavaCode\BooleanDemo, java.runtime.version=1.7.0_25-b17, java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment, java.endorsed.dirs=C:\Program Files\Java\jre7\lib\endorsed, os.arch=x86, java.io.tmpdir=R:\TEMP\, line.separator=
, java.vm.specification.vendor=Oracle Corporation, user.variant=, os.name=Windows XP, sun.jnu.encoding=MS950, java.library.path=C:\Program Files\Java\jre7\bin;C:\WINDOWS\Sun\Java\bin;C:\WINDOWS\system32;C:\WINDOWS;C:/Program Files/Java/jre7/bin/client;C:/Program Files/Java/jre7/bin;C:/Program Files/Java/jre7/lib/i386;C:\Ruby200\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\Program Files\Microsoft SQL Server\90\Tools\binn\;C:\Program Files\Perforce;C:\Program Files\Java\jdk1.7.0_07\bin;C:\Program Files\Android\android-sdk\tools;C:\Program Files\Android\android-sdk\platform-tools;C:\Program Files\apache-ant-1.9.1\bin;C:\Program Files\Perforce\Server;C:\Program Files\eclipse-SDK-4.2-win32;;., java.specification.name=Java Platform API Specification, java.class.version=51.0, sun.management.compiler=HotSpot Client Compiler, os.version=5.1, user.home=C:\Documents and Settings\Josh, user.timezone=, java.awt.printerjob=sun.awt.windows.WPrinterJob, file.encoding=UTF-8, java.specification.version=1.7, java.class.path=C:\Documents and Settings\Josh\JavaCode\BooleanDemo\bin, user.name=Josh, java.vm.specification.version=1.7, sun.java.command=BooleanDemo, java.home=C:\Program Files\Java\jre7, sun.arch.data.model=32, user.language=zh, java.specification.vendor=Oracle Corporation, awt.toolkit=sun.awt.windows.WToolkit, java.vm.info=mixed mode, sharing, java.version=1.7.0_25, java.ext.dirs=C:\Program Files\Java\jre7\lib\ext;C:\WINDOWS\Sun\Java\lib\ext, sun.boot.class.path=C:\Program Files\Java\jre7\lib\resources.jar;C:\Program Files\Java\jre7\lib\rt.jar;C:\Program Files\Java\jre7\lib\sunrsasign.jar;C:\Program Files\Java\jre7\lib\jsse.jar;C:\Program Files\Java\jre7\lib\jce.jar;C:\Program Files\Java\jre7\lib\charsets.jar;C:\Program Files\Java\jre7\lib\jfr.jar;C:\Program Files\Java\jre7\classes, java.vendor=Oracle Corporation, file.separator=\, java.vendor.url.bug=http://bugreport.sun.com/bugreport/, sun.io.unicode.encoding=UnicodeLittle, sun.cpu.endian=little, sun.desktop=windows, sun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+mmx pentium i486 i386 i86}
======================================
Java(TM) SE Runtime Environment
private boolean convertToBoolean(String value) {
if ("1".equalsIgnoreCase(value) ||
"yes".equalsIgnoreCase(value) ||
"true".equalsIgnoreCase(value)||
"on".equalsIgnoreCase(value))
return true;
return false;
}
<textview android:id="@+id/tv" android:layout_height="wrap_content" android:layout_width="wrap_content" android:scalex="-1f" android:scaley="-1f" android:text="@string/hello_world"/>設定後的結果如下圖所示：
TextView tv = (TextView)findViewById(R.id.tv); tv.setScaleX(-1f); tv.setScaleY(-1f);
<application ... android:theme= "@android:style/Theme.NoTitleBar.Fullscreen" > </application>
<activity android:theme= "@android:style/Theme.NoTitleBar.Fullscreen" > ... </activity>
requestWindowFeature(Window.FEATURE_NO_TITLE);
//fullscreen
this.getWindow().setFlags(
WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN );
setTheme(android.R.style.Theme_NoTitleBar_Fullscreen);
package com.example.fullscreendemo; import android.os.Bundle; import android.view.Gravity; import android.view.Window; import android.view.WindowManager; import android.widget.LinearLayout; import android.app.Activity; public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // Set Full screen in Java file method 1 //turn off the window's title bar //requestWindowFeature(Window.FEATURE_NO_TITLE); // fullscreen //this.getWindow().setFlags(
// WindowManager.LayoutParams.FLAG_FULLSCREEN, // WindowManager.LayoutParams.FLAG_FULLSCREEN);
LinearLayout ll =
new LinearLayout(getApplicationContext());
ll.setOrientation(LinearLayout.VERTICAL);
ll.setGravity(/*Gravity.CENTER*/Gravity.TOP);
LinearLayout.LayoutParams lp =
new LinearLayout.LayoutParams(
LinearLayout.LayoutParams.MATCH_PARENT,
LinearLayout.LayoutParams.MATCH_PARENT);
ll.setLayoutParams(lp);
ll.setBackgroundResource(
R.drawable.background2);
// Set Full screen in Java file method 2
setTheme(
android.R.style.Theme_NoTitleBar_Fullscreen);
setContentView(ll); }
}
package com.example.screenresolutiondemo;
import android.app.Activity;
import android.content.res.Resources;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.widget.TextView;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView tv = new TextView(getApplicationContext());
Resources res = getResources();
DisplayMetrics metrics = res.getDisplayMetrics();
tv.setText("螢幕資訊:\n");
tv.append("寬:" + metrics.widthPixels + "\n");
tv.append("高:" + metrics.heightPixels + "\n");
tv.append("DPI:" + metrics.densityDpi + "\n");
if (metrics.widthPixels < metrics.heightPixels) {
tv.append("目前是直式\n");
} else {
tv.append("目前是橫式\n");
}
setContentView(tv);
}
}
範例結果：
public class HelloWorld {
public static void main(String [] args) {
System.out.println("Hello World!");
}
}
設定好之後，就可以透過下面的方式張貼出有 Highlight Syntax 的程式碼了。
<pre class="prettyprint code"> 輸入想顯示的程式碼 </pre>