从一个类调用一个方法到另一个类

Question

I wrote this method in one class for checking if a table exists in my database or not:

public boolean DoesTableExist(String tableName) {
    SQLiteDatabase mDatabase = openOrCreateDatabase("privdb",MODE_PRIVATE,null);

    Cursor cursor = mDatabase.rawQuery("select DISTINCT tbl_name from sqlite_master where tbl_name = '"+tableName+"'", null);
    if(cursor!=null) {
        if(cursor.getCount()>0) {
            cursor.close();
            return true;

        }
        cursor.close();
    }

    return false;
}

I now would like to call this method in other activites or classes, without writing the same code everytime in every file. How can I do that?

Answer 1

Normally you'd invoke methods in another class by instantiating the class and then calling the method. For instance, if the class that the method was in was named DatabaseUtils , you'd do the following:

DatabaseUtils dbUtils = new DatabaseUtils();
boolean exists = dbUtils.doesTableExist("tablename"); 
                      // ^ note that the normal Java convention is to make
                      //   method names start with a lower case letter

However, this has the feel of a "utility" method (the object presumably doesn't store any state, and thus there's no real need to instantiate it), and a common way of handling those are to make the method static, like so:

public static boolean doesTableExist(String tableName) {

That causes the method to exist on the class definition itself, instead of on a specific instance of a class, and allows you to do the following:

boolean exists = DatabaseUtils.doesTableExist("tablename");

Answer 2

Make a instance of the class that method is in, create a get method and call method through that instance. Other option is to define method as a static and call it that way.

Instance:

ClassName classInstance = new ClassName();
classInstance.getMethod(parametar);

Static:

ClassName.methodName(parametar);

Here is more about that: How to call a method in another class in Java?

Answer 3

That would be a nice one to put in a "utils" package. Create a new Java package named "utils", add a new class, such as "DatabaseUtils.java" and make it look something like this:

    public class DatabaseUtils {

    public boolean DoesTableExist(String tableName) {
                SQLiteDatabase mDatabase = openOrCreateDatabase("privdb",MODE_PRIVATE,null);

                Cursor cursor = mDatabase.rawQuery("select DISTINCT tbl_name from sqlite_master                 where tbl_name = '"+tableName+"'", null);
                if(cursor!=null) {
                    if(cursor.getCount()>0) {
                        cursor.close();
                        return true;
                    }
                    cursor.close();
                }
                return false;
            }

    }

Now you can reference it from other places like this:

    public class main {

            public static void main(String [] args)
                    boolean isTableExisting = DatabaseUtils.doesTableExist("yourTableName")
                    System.out.println("Table existing? " + isTableExisting);
            }
    }

Probably you'll need to pass the name of the database as well ("privdb") or reference to where the method "openOrCreateDatabase(...)" is located.

When your database does exist, it will print "Table existing? true"