- Learning Java by Building Android Games
- John Horton
- 488字
- 2021-07-23 19:02:27
If they come over the bridge shoot them
As we saw in the previous chapter, operators are used in determining whether and how often a loop should execute the code in its body.
We can now take things a step further. Let's look at putting the most common operator == to use with the Java if and else keywords then we can start to see the powerful yet fine control that they offer us.
We will use if and a few conditional operators along with a small story to demonstrate their use. Next follows a made up military situation that is kind of game-like in its nature.
The captain is dying and, knowing that his remaining subordinates are not very experienced, he decides to write a Java program to convey his last orders after he has died. The troops must hold one side of a bridge while awaiting reinforcements.
The first command the captain wants to make sure his troops understand is this:
If they come over the bridge, shoot them.
So how do we simulate this situation in Java? We need a Boolean variable isComingOverBridge. The next bit of code assumes that the isComingOverBridge variable has been declared and initialized to either true or false.
We can then use if like this.
if(isComingOverBridge){
// Shoot them
}
If the isComingOverBridge Boolean is true the code inside the opening and closing curly braces will run. If not the program continues after the if block and without running the code within it.
Else do this instead
The captain also wants to tell his troops what to do (stay put) if the enemy is not coming over the bridge.
Now we introduce another Java keyword, else. When we want to explicitly do something when the if does not evaluate to true, we can use else.
For example, to tell the troops to stay put if the enemy is not coming over the bridge we could write this code:
if(isComingOverBridge){
// Shoot them
}else{
// Hold position
}
The captain then realized that the problem wasn't as simple as he first thought. What if the enemy comes come over the bridge, but has too many troops? His squad would be overrun. So, he came up with this code (we'll use some variables as well this time.):
boolean isComingOverBridge;
int enemyTroops;
int friendlyTroops;
// Code that initializes the above variables one way or another
// Now the if
if(isComingOverBridge && friendlyTroops > enemyTroops){
// shoot them
}else if(isComingOverBridge && friendlyTroops < enemyTroops) {
// blow the bridge
}else{
// Hold position
}
The above code has three possible paths of execution. First, if the enemy is coming over the bridge and the friendly troops are greater in number.
if(isComingOverBridge && friendlyTroops > enemyTroops)
Second, if the enemy troops are coming over the bridge and outnumber the friendly troops.
else if(isComingOveBridge && friendlyTroops < enemyTroops)
Then the third and final possible outcome which will execute if neither of the others is true is captured by the final else without an if condition.
Tip
Reader challenge
Can you spot a flaw in the above code? One that might leave a bunch of inexperienced troops in complete disarray? The possibility of the enemy troops and friendly troops being exactly equal in number has not been handled explicitly and would, therefore, be handled by the final else which is meant for when there are no enemy troops. I guess any self-respecting captain would expect his troops to fight in this situation and he could have changed the first if statement to accommodate this possibility.
if(isComingOverBridge && friendlyTroops >= enemyTroops)
And finally, the captain's last concern was that if the enemy came over the bridge waving the white flag of surrender and was promptly slaughtered, then his men would end up as war criminals. The Java code needed was obvious. Using the wavingWhiteFlag Boolean variable he wrote this test.
if (wavingWhiteFlag){
// Take prisoners
}
But where to put this code was less clear. In the end, the captain opted for the following nested solution and changing the test for wavingWhiteFlag to logical NOT, like this:
if (!wavingWhiteFlag){
// not surrendering so check everything else
if(isComingOverTheBridge && friendlyTroops >= enemyTroops){
// shoot them
} else if (isComingOverTheBridge && friendlyTroops < enemyTroops) {
// blow the bridge
}
}else{
// This is the else for our first if
// Take prisoners
}
// Holding position
This demonstrates that we can nest if and else statements inside of one another to create quite deep and detailed decisions.
We could go on making more and more complicated decisions with if and else but what we have seen is more than sufficient as an introduction. It is probably worth pointing out that very often there is more than one way to arrive at a solution to a problem. The right way will usually be the way that solves the problem in the clearest and simplest manner.
Let's look at some other ways to make decisions in Java and then we can put them all together in an app.
Switching to make decisions
We have seen the vast and virtually limitless possibilities of combining the Java operators with if and else statements. But sometimes a decision in Java can be better made in other ways.
When we must decide based on a clear list of possibilities that don't involve complex combinations, then the switch is usually the way to go.
We start a switch decision like this.
switch(argument){
}
In the previous example, an argument could be an expression or a variable. Within the curly braces {} we can make decisions based on the argument with case and break elements.
case x: // code to for case x break; case y: // code for case y break;
You can see in the previous example each case states a possible result and each break denotes the end of that case and the point at which no further case statements will be evaluated.
The first break encountered breaks out of the switch block to proceed with the next line of code after the closing brace } of the entire switch block.
We can also use default without a value to run some code in case none of the case statements evaluate to true. Like this:
default:// Look no value // Do something here if no other case statements are true break;
Let's look at a complete example of a switch in action.
Switch example
Let's pretend we are writing an old-fashioned text adventure game, the kind of game where the player types commands like "Go East", "Go West" and "Take Sword", etc. In this case, switch could handle that situation like this example code and we could use default to handle the player typing a command which is not specifically handled.
// get input from user in a String variable called command
String command = "go east";
switch(command){
case "go east":
Log.d("Player: ", "Moves to the east" );
break;
case "go west":
Log.d("Player: ", "Moves to the East" );
break;
case "go north":
Log.d("Player: ", "Moves to the North" );
break;
case "go south":
Log.d("Player: ", "Moves to the South" );
break;
case "take sword":
Log.d("Player: ", "Takes the silver sword" );
break;
// more possible cases
default:
Log.d("Message: ", "Sorry I don't speak Elvish" );
break;
}
Depending upon the initialization of command it might be specifically handled by one of the case statements. Otherwise, we get the default Sorry I don't speak Elvish.
If we had a lot of code to execute for a particular case, we could contain it all in a method. Perhaps, like this next piece of code, I have highlighted the new line.
default:
goWest();
break;
Of course, we would then need to write the new goWest method. Then, when the command variable was initialized to "go west", the goWest method would be executed and execution would return to the break statement which would cause the code to continue after the switch block.