package com.jignesh;
import java.io.File;
public class DeleteFileExample
{
public static void main(String[] args)
{
try{
File file = new File("c:\\jigneshExample.txt");
if(file.delete()){
System.out.println(file.getName() + " is deleted!");
}else{
System.out.println("Delete operation is failed.");
}
}catch(Exception e){
e.printStackTrace();
}
}
}DBObject doc = collection.findOne();
System.out.println(dbObject); { "_id" : { "$oid" : "id"} , "number" : 1 , "name" : "jignesh-1"}
DBCursor cursor = collection.find();
while(cursor.hasNext()) {
System.out.println(cursor.next());
} 1.3 Get single field from matched document.{ "_id" : { "$oid" : "id"} , "number" : 1 , "name" : "jignesh-1"} { "_id" : { "$oid" : "id"} , "number" : 2 , "name" : "jignesh-2"} { "_id" : { "$oid" : "id"} , "number" : 3 , "name" : "jignesh-3"}
BasicDBObject allQuery = new BasicDBObject();
BasicDBObject fields = new BasicDBObject();
fields.put("name", 1);
DBCursor cursor = collection.find(allQuery, fields);
while (cursor.hasNext()) {
System.out.println(cursor.next());
} { "_id" : { "$oid" : "id"} , "name" : "jignesh-1"} { "_id" : { "$oid" : "id"} , "name" : "jignesh-2"} { "_id" : { "$oid" : "id"} , "name" : "jignesh-3"}
equals() and hashcode() are not implemented, e.g.
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| import java.util.Map;public class MemLeak {public final String key;public MemLeak(String key) { this.key = key;} public static void main(String args[]) { try { Map map = System.getProperties(); for(;;) { map.put(new MemLeak("key"), "value"); } } catch(Exception e) { e.printStackTrace(); } }} |
'./visualvm' (or visualvm.exe on windows).
(sudo yum update).
If not…
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| long start = System.currentTimeMillis();.. //your code..long end = System.currentTimeMillis();System.out.println("Run time: " + Long.toString(end - start));System.out.println(printGCStats());public static String printGCStats() { long totalGarbageCollections = 0; long garbageCollectionTime = 0; for (GarbageCollectorMXBean gc : ManagementFactory.getGarbageCollectorMXBeans()) { long count = gc.getCollectionCount(); if (count >= 0) { totalGarbageCollections += count; } long time = gc.getCollectionTime(); if (time >= 0) { garbageCollectionTime += time; } } return "Garbage Collections: " + totalGarbageCollections + "\n" + "Garbage Collection Time (ms): " + garbageCollectionTime;} |
Line nn: class or interface declaration expectedwhen, for example, you capitalise the keyword class.
Line nn: ';' expectedWhen this happens the solution is to split the string into two, making sure that neither string has a new line in it, and concatenate them with +. Thus you might replace:
String s = "A very long string which just happens to go over the end of a line and causes a problem with the compiler";with:
String s = "A very long string which just happens to go over the end "+ "of a line and causes a problem with the compiler"
objSend.carryOut()rather than:
objSend.carryOutThe compiler will usually emit an error message of the form:
Line nn: Invalid expression statement
Line nn: Class xxxx not found in type declarationDon't forget, though, that java.lang is imported automatically and, hence, does not need an import statement.
int result = Math.abs(value);rather than:
int result = value.abs();This gives rise to a variety of syntax errors. The most common one is of the form:
Line nn: Method yyyy not found in class xxxx.where yyyy is the name of the method and xxxx is the name of the class within which it is called.
Line nn: Class xxxx not found in type declaration.where xxxx is the name of the class which has not been given the correct capitalisation.
Line nn: Undefined variable: xxxxwhere xxxx is the name of the variable which has been mistyped.
ClassName.MethodName(Argument(s))A common error is to forget the class name. If you do, then you will get an error message of the form:
Line nn: '}' expected
public void tryIt(int a, int b, URL c)A common error that programmers from other languages make is to forget to prefix every argument with its type. For example, an erroneous version of the definition above would be:
public void tryIt(int a, b URL c)This type of error will give rise to error messages of the form:
Line nn: Identifier expected
int newVal = destination. tryIt(arg1, arg2)where the arguments are ints which have been declared somewhere. A common mistake is to write this as:
int newVal = tryIt(destination, arg1,arg2)This gives rise to error messages of the form:
Line nn: ')' expected
if(newObj1 == newObj2){
...
}
will execute the code denoted by the three dots only if the
first object occupies the same address as the second object. If the
objects occupied different addresses, but still had the same values for
their instance variables, then it would evaluate to false. Unfortunately
this does not give rise to any syntax errors, but will show up when any
program containing the error is executed. Line nn: Invalid method declaration; return type required
Line nn: Return required at end of xxxxwhere xxxx is the method which does not contain the return.
Line nn: Variable xx in class xxxx not accessible from class yyyywhere xx is the private variable, xxxx is the class in which it is defined and class yyyy is the class in which it is referred to.
Line nn: Incompatible type for declaration can't convert xxxx to yyyy
y = ++xis executed, then y and x both become 46. If the statement
y = x++is executed, then y becomes 45, while x becomes 46. These errors will not be signalled at compile time, but will emerge during run time.
arrayVariable.size()or
stringVariable.sizethen the first would generate an error message of the form:
Line nn: Method size() not found in class java.lang.Objectand the second would generate an error message of the form:
Line nn: No variable size defined in java.lang.String
Line nn: No constructor matching xxxx found in class yyyywould be generated, where xxxx is the signature of the constructor that you have tried using and yyyy is the name of the class which it should have been defined in.
Line nn: Method xxxx not found in yyyywhere xxxx is the name of the constructor and its arguments and yyyy is the name of the class which it is defined in. The solution is to use the this keyword.
int [,] arrayVariable = new [10,20] intThis is illegal and will give rise to an errors of the form:
Line nn: Missing termand:
Line nn: ']' expectedYou can implement many-dimensional arrays in Java, but they are treated like single-dimension arrays which contain single-dimensional arrays which contain single dimension arrays, etc.
Vector vec = new Vector(); vec.addElement(12);If you write code such as that shown above then it will give rise to syntax errors of the form:
Line nn: No method matching xxxx found in yyyywhere xxxx is the name of the method which is used and yyyy is the name of the class which expects an Object. The solution is to use the object wrapper classes found in java.lang to convert them to objects.
int y = 22; Integer x = y;will give rise to an error message of the form:
Line nn: Incompatible type for declaration. Can't convert xxxx to yyyywhere xxxx and yyyy are the classes involved.
public static void main (String []args){
If you mistype any part of this line or miss out a keyword, then a
run-time error will be generated. For example, if you miss out the keyword
static then an error message of the form: Exception in thread main.....
val@NonNull@Cleanupclose() methods safely with no hassle.@Getter / @Setterpublic int getFoo() {return foo;} again.@ToStringtoString for you!@EqualsAndHashCodehashCode and equals implementations from the fields of your object.@NoArgsConstructor, @RequiredArgsConstructor and @AllArgsConstructor@Data@ToString, @EqualsAndHashCode,
@Getter on all fields, and @Setter on all non-final fields, and @RequiredArgsConstructor!@Value@Builder@SneakyThrows@Synchronizedsynchronized done right: Don't expose your locks.@Getter(lazy=true)@Log<accordion></accordion>, <grid></grid>, <lightbox></lightbox>, etc.) that we never have to manipulate the DOM to simulate them. All that our app needs to do is to assign attributes to elements to get any functionality out of the box.<myticker></myticker><div data-myticker></div><div class="myticker"></div>broadcast() will send a message to all children controllers, while emit() will send a message to all ancestors.