There are several differences between HashMap and Hashtable in Java:
Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.
Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.
Java is always pass-by-value. Unfortunately, when we deal with objects we are really dealing with object-handles called references which are passed-by-value as well. This terminology and semantics easily confuse many beginners.
It goes like this:
public static void main(String[] args) {
Dog aDog = new Dog("Max");
Dog oldDog = aDog;
// we pass the object to foo
foo(aDog);
// aDog variable is still pointing to the "Max" dog when foo(...) returns
aDog.getName().equals("Max"); // true
aDog.getName().equals("Fifi"); // false
aDog == oldDog; // true
}
public static void foo(Dog d) {
d.getName().equals("Max"); // true
// change d inside of foo() to point to a new Dog instance "Fifi"
d = new Dog("Fifi");
d.getName().equals("Fifi"); // true
}
In the example above aDog.getName() will still return "Max". The value aDog within main is not changed in the function foo with the Dog "Fifi" as the object reference is passed by value. If it were passed by reference, then the aDog.getName() in main would return "Fifi" after the call to foo.
Likewise:
public static void main(String[] args) {
Dog aDog = new Dog("Max");
Dog oldDog = aDog;
foo(aDog);
// when foo(...) returns, the name of the dog has been changed to "Fifi"
aDog.getName().equals("Fifi"); // true
// but it is still the same dog:
aDog == oldDog; // true
}
public static void foo(Dog d) {
d.getName().equals("Max"); // true
// this changes the name of d to be "Fifi"
d.setName("Fifi");
}
In the above example, Fifi is the dog's name after call to foo(aDog) because the object's name was set inside of foo(...). Any operations that foo performs on d are such that, for all practical purposes, they are performed on aDog, but it is not possible to change the value of the variable aDog itself.
For more information on pass by reference and pass by value, consult the following SO answer: https://stackoverflow.com/a/430958/6005228. This explains more thoroughly the semantics and history behind the two and also explains why Java and many other modern languages appear to do both in certain cases.
Best Answer
The short answer to your question is that "opaque" is defined in English as completely non-transparent. Therefore an opaque component is one which paints its entire rectangle, and every pixel is not at all translucent to any degree.
However, the Swing component opacity API is one of those mis-designed and therefore often mis-used APIs.
What's important to understand is that
isOpaqueis a contract between the Swing system and a particular component. If it returns true, the component guarantees to non-translucently paint every pixel of its rectangular area. This API should have been abstract to force all component writers to consider it. TheisOpaqueAPI is used by Swing's painting system to determine whether the area covered by a given component must be painted for components which overlap it and which are behind it, including the component's container and ancestors. If a component returns true to this API, the Swing system may optimize painting to not paint anything in that area until invoking the specific component's paint method.Because of contractual implication of
isOpaque, the APIsetOpaqueshould not exist, since it is actually incorrect for anything external to callsetOpaquesince, in turn, the external thing can't know whether the component in question will (or even can) honor it. Instead, isOpaque should have been overridden by each concrete component to return whether it actually is, in fact, opaque given its current properties.Because the
setOpaqueAPI does exist, many components have mis-implemented it (quite understandably) to drive whether or not they will paint their "background" (for example JLabel and JPanel filling with their background color). The effect of this is to create an impression with users of the API to think thatsetOpaquedrives whether or not that background should paint, but it doesn't.Furthermore, if, say, you wish to paint a JLabel with a translucent background you need to set a background color with an alpha value, and do
setOpaque(true), but it's not actually opaque - it's translucent; the components behind it still need to paint in order for the component to render properly.This problem was exposed in a significant way with the Java 6's new Nimbus Look & Feel. There are numerous bug reports regarding transparent components filed against Nimbus (see stack overflow question Java Nimbus LAF with transparent text fields). The response of the Nimbus development team is this:
So, in summary, you should not use
setOpaque. If you do use it, bear in mind that the combination of some Look & Feels and some components may do "surprising" things. And, in the end, there is actually no right answer.