I tried writing a small program, but it will not compile. Eclipse shows the following errors on the last line:
Syntax error on token "200000", delete this token
import java.util.ArrayList;
public class Books{
ArrayList<String> booksDB = new ArrayList<String>();
booksDB.ensureCapacity(200000); //Compilation Errors
}
public class Books{
ArrayList<String> booksDB = new ArrayList<String>();
booksDB.ensureCapacity(200000); //Compilation Errors
}
You might be under the impression that this code is executing sequentially (from top to bottom) but that is not the case. What you are doing is creating a private instance field named booksDB that every instance of Books will carry.
Most likely, you want that code to go in the Books constructor (which gets called whenever a new instance of Books is created. Try the following:
public class Books {
private ArrayList<String> booksDB; // this is a field of the Books class
// when we create a new Books instance, we will initialize the booksDB field
public Books() {
booksDB = new ArrayList<String>();
booksDB.ensureCapacity(20000);
}
}
Then use as follows:
public class Main {
public static void main(String[] args) {
Books b = new Books(); // the Books object is constructed, and its private field booksDB is initialized as we specified.
}
}
Except for declarations and imports, all code in Java must reside inside a method (or, as discussed below, inside initializers, which are essentially methods too):
import java.util.ArrayList;
public class Books {
ArrayList<String> booksDB = new ArrayList<String>();
public void doSomething() {
booksDB.ensureCapacity(200000); //Compilation Errors
}
}
Since you seem to be initializing the contents of the object, you might want to put that line in a constructor, which is a method that is automatically run on newly-created objects:
import java.util.ArrayList;
public class Books {
ArrayList<String> booksDB = new ArrayList<String>();
public Books() {
booksDB.ensureCapacity(200000); //Compilation Errors
}
}
static {...}?
You can use constructor of ArrayList that accepts initial capacity like below
import java.util.ArrayList;
public class Books{
ArrayList<String> booksDB = new ArrayList<String>(200000);
}
Access variables ( booksDB ) only from methods. That's all you have to do.
