如何退出我的主要方法

Question

这是我的主要方法。我有一个草食动物和我的植物的ArrayList，画布大小为（1000 x 1000）。我让它们做的是：所有的草食动物都吃离它们最近的植物。我知道需要做的是，一旦所有的植物都被吃掉了，他们的主要方法应该停止并打印一些统计数据。我所知道的是，在我的草食动物开始寻找下一株植物之前，我需要退出，因为这会给我一个错误，因为arrayList现在是空的。但是，如果我在findNearestWithinFiftyMeters方法之前使用"System.exit（0）；"，那么它将执行我的其余代码。我如何让它退出，但仍然打印我的统计数据？

import java.util.ArrayList;
public class TestDriver {
    public static void main(String[] args){
        //Set the canvas size to (1000,1000) and x and y scale to (0,1000)
        StdDraw.setCanvasSize(1000,1000);
        StdDraw.setXscale(0.0 , 1000.0);
        StdDraw.setYscale(0.0 , 1000.0);
        //ArrayList for the Plants
        ArrayList<Plant> thePlants = new ArrayList<Plant>();
        //Create 300 Plants
        for (int i=0; i<300; i++){
            thePlants.add(new Plant (1000 * Math.random(),1000 * Math.random()));
        }

        //ArrayList for the Herbivores
        ArrayList<Herbivore> theHerbivores = new ArrayList<Herbivore>();
        //Create 20 Herbivores
        for (int i=0; i<20; i++){
            theHerbivores.add(new Herbivore (1000 * Math.random(), 1000 * Math.random()));
        }
        //Draw Graphics:
        while(theHerbivores.size() > 0){
            //Clears the board
            StdDraw.clear();
            //created Herbivores of size
            for(int i=0; i<theHerbivores.size(); i++){  
                //Draws the herbivore at position i
                theHerbivores.get(i).draw();
                if(thePlants.size() == 0){
                 StdDraw.clear();
                }
                //Finds the plant that is closest to the herbivore
                Plant closest = findNearestWithinFiftyMeters(thePlants , theHerbivores.get(i));
                if(closest != null){
                    //IF the closest plant is NOT null then move towards closest plant
                    theHerbivores.get(i).moveToward(closest.getX(), closest.getY() );
                    //now that we have the closest plant in regards to this herbivore
                    //we want it to move to the plant
                    theHerbivores.get(i).createRandomTarget();
                    //Reset the target each time it finds a new plant to chew
                }
                else{
                    //if it IS null
                    //Walk in a random direction (towards the imaginary plant)
                    theHerbivores.get(i).move();
                }
            }
            //Draws plants
            for(Plant p: thePlants){
                p.draw();
            }
            //Check for any herbivores that have moved off screen
            for(int i=0; i < theHerbivores.size(); i++){
                //if an herbivore moved too far left or right move to other side of screen
                if(theHerbivores.get(i).getX()>1000){
                    theHerbivores.get(i).setX(0);
                }
                else if(theHerbivores.get(i).getX()<0){
                    theHerbivores.get(i).setX(1000);
                }               
                //if an herbivore moved too far up or down
                if(theHerbivores.get(i).getY()>1000){
                    theHerbivores.get(i).setY(0);
                }
                else if(theHerbivores.get(i).getY()<0){
                    theHerbivores.get(i).setY(1000);
                }
            }

            //looping through all the plants to remove plants that have been eaten
            for(int i=0; i< theHerbivores.size(); i++){
                for(int j = 0; j < thePlants.size(); j++){
                    if(thePlants.get(j).distanceTo(theHerbivores.get(i).getX(),theHerbivores.get(i).getY()) < 3){
                        thePlants.remove(j);
                        theHerbivores.get(i).eat();
                        //INCREMENT HERBIVORE EATEN COUNT
                    }
                }
                StdDraw.show(1);
            }
            stepCounter++;
        }//end while loop
        System.out.println(stepCounter + " steps done in this simulation");
        long estimatedTime = System.nanoTime() - startTime;
        System.out.println(estimatedTime + "Length of time simulation used" );
        for(int i=0; i<theHerbivores.size(); i++){
            System.out.println("Herbivore # " + i + "X: " + theHerbivores.get(i).getX() + "Y: " + theHerbivores.get(i).getY() + " EATEN: "+ theHerbivores.get(i).getEatCount());
        }
        return;
    } // end of main method

    static long startTime = System.nanoTime(); 
    static int stepCounter = 0;
    public static Plant findNearestWithinFiftyMeters(ArrayList<Plant> thePlants , Herbivore eater){
        //plant variable for storage to find closest plant to that herbivore
        Plant closest = thePlants.get(0);
        for(int i=0; i<thePlants.size(); i++){
            if(eater.distanceTo(closest.getX(), closest.getY()) > eater.distanceTo(thePlants.get(i).getX(),thePlants.get(i).getY()) ){
                //if the plant in closest variable is farther away than the 
                //plant in index 'i' then replace the plant in closest with the plant in index 'i'
                closest = thePlants.get(i);
            }
        }
        if(eater.distanceTo(closest.getX(),closest.getY()) > 50){
            //if distance is greater than 50(herbivore sight range) then set closest equal to null
            closest=null;
        }
        return closest;
    }   
} // end of class

Answer 1

您可以更改while循环，使其仅在有草食动物和植物的情况下运行。

while(theHerbivores.size() > 0 && thePlants.size() > 0){

Answer 2

重点是不是退出main方法，而是退出进食动作的循环以打印一些统计数据。一个很好的方法是改变循环条件，使其在不需要时终止：

while(theHerbivores.size() > 0 && thePlants.size() > 0) {
 //Rest of code....
}

这将导致在outter循环之后执行代码的其余部分。

作为问题的补充，您可以在需要时使用break语句进入循环外，通常与循环内的条件if/else if/else语句结合使用。System.exit(0)将导致您的程序终止的实际力，如前所述，这是而不是您想要的。

Answer 3

好吧，据我所知，您正试图在主方法结束后打印一些信息，而不终止整个程序。如果这是正确的，您应该创建一个打印这些统计信息的方法。然后在主方法的末尾调用此方法。这样，主方法在调用此方法后结束，但程序将继续运行，直到打印出统计信息。

这里有一个例子：

public static void main(String[] args) {
    //everything in the main method
    //the end of the main method
    printStatistics();
    //main method terminates here but the program runs until the printStatistics() method finishes running
}
private void pintStatistics() {
    System.out.println(/*insert statistics here*/);
    //entire program terminates here
}