1.     if (finished = true) {

must be

1.     if (finished == true) {

but rather:

1.     if (myhumans[i].finished == false) {

?
I dunno want you want there.


when you are referring to the humans

1.   void finished() {
2.       finished = true;
3.     }

where is var finished defined?

do you have

  if (myhumans[i].finished() == false) {


or  

if (!myhumans[i].finished()) {

1.   background(img);

must be 1st line in draw()


here


alien myalien;
humans[] myhumans;
int totalHumans = 0;
Timer mytimer;

boolean gameOver = false;

int score = 0;
int level = 1;
int lives = 10;
int levelCounter = 0;

PFont f;

PImage img;

void setup() {
  size(800, 600);
  smooth();
  img = loadImage("space_cat.jpg");
  background(0);
  myalien = new alien();
  myhumans = new humans[50];
  mytimer = new Timer(3000);
  mytimer.start();
  f = createFont("Arial", 12, true);
}



void draw() {
  background(0);
  if (gameOver) {
    textFont(f, 48);
    textAlign(CENTER);
    fill(0);
    text("GAME OVER", WIDTH/2, HEIGHT/2);
  }
  else {
    myalien.display();
    myalien.move();
    if (mytimer.isFinished()) {
      if (totalHumans < myhumans.length) {
        myhumans[totalHumans] = new humans();
        totalHumans++;
      }
      mytimer.start();
    }
    for (int i = 0; i < totalHumans; i++) {
      if (!myhumans[i].finished) {
        myhumans[i].move();
        myhumans[i].drawBody();
        if (myhumans[i].reachedBottom()) {
          levelCounter++;
          myhumans[i].finished();
          lives--;
          if (lives <= 0) {
            gameOver = true;
          }
        }
        if (myalien.intersect(myhumans[i])) {
          myhumans[i].finished();
          levelCounter++;
          score++;
        }
      }
    }
    if (levelCounter >= myhumans.length) {
      level++;
      levelCounter = 0;
      lives = 10;
      mytimer.setTime(constrain(300-level*25, 0, 300));
      totalHumans = 0;
    }
    textFont(f, 14);
    fill(0);
    text("Lives left: "+ lives, 10, 20);
    rect(10, 24, lives*10, 10);

    text("Level: "+ level, 300, 20);
    text("Score: "+ score, 300, 40);
  }
}



//-------------------------------

class Timer {

  int savedTime;
  int totalTime;

  Timer(int tempTotalTime) {
    totalTime = tempTotalTime;
  }

  void setTime(int t) {
    totalTime = t;
  }

  void start() {
    savedTime = millis();
  }

  boolean isFinished() {
    int passedTime = millis() - savedTime;
    if (passedTime > totalTime) {
      return true;
    }
    else {
      return false;
    }
  }
}




//-----------------------------


class alien {

  float bodyXLoc;
  float bodyYLoc;
  int bodyWidth;
  int bodyHeight;

  int headWidth;
  float headHeight;
  float headXLoc;
  float headYLoc;
  int eyeWidth;
  int eyeHeight;

  int speed;

  int direction;


  alien () {
    bodyXLoc = 450;
    bodyYLoc = 300;
    bodyWidth = 50;  //changes width of the body
    bodyHeight = 100;  //changes height of the body
    headHeight = 100;
    eyeWidth = 30;  //changes the width of the eyes
    eyeHeight = 30;  //changes the height of the eyes
    speed = 5;  //changes how fast the alien moves
    direction = 1;
  }



  //  void setLocation(float tempX, float tempY)  {
  //    bodyXLoc = tempX;
  //    bodyYLoc = tempY;
  //  }

  void display() {
    //Draw body
    stroke(0, 0, 0);
    strokeWeight(1);
    fill(137, 242, 204);  //defines the colour for the alien's body
    rect(bodyXLoc, bodyYLoc, bodyWidth, bodyHeight);  //draws the aliens body
    stroke(255, 255, 255);  //defines the colour of the alien's arms
    strokeWeight(8);  //defines the thickness of the alien's arms
    line(bodyXLoc+51, bodyYLoc+30, bodyXLoc+100, bodyYLoc+70);  //draws the arms
    line(bodyXLoc+51, bodyYLoc+50, bodyXLoc+100, bodyYLoc+90);
    line(bodyXLoc+51, bodyYLoc+70, bodyXLoc+100, bodyYLoc+110);
    line(bodyXLoc, bodyYLoc+30, bodyXLoc-50, bodyYLoc+70);
    line(bodyXLoc, bodyYLoc+50, bodyXLoc-50, bodyYLoc+90);
    line(bodyXLoc, bodyYLoc+70, bodyXLoc-50, bodyYLoc+110);
    line(bodyXLoc+10, bodyYLoc+100, bodyXLoc+10, bodyYLoc+150);
    line(bodyXLoc+25, bodyYLoc+100, bodyXLoc+25, bodyYLoc+150);
    line(bodyXLoc+40, bodyYLoc+100, bodyXLoc+40, bodyYLoc+150);

    //Draw head
    fill(137, 242, 204);  //defines the colour of the alien's head
    stroke(0);  //defines the colour of the outline of the alien's head
    strokeWeight(1);  //thins the line around the head
    headWidth = bodyWidth*2;  //defines the width of the head
    headXLoc = bodyXLoc+25;  //defines the location where the head is drawn
    headYLoc = bodyYLoc+5;  // ""
    ellipse(headXLoc, headYLoc, headWidth, headHeight);  //draws the head
    fill(255, 255, 255);  //defines the colour of the eyes
    ellipse(headXLoc-10, headYLoc-15, eyeWidth+10, eyeHeight+10);  //draws the first eye
    ellipse(headXLoc+25, headYLoc-5, eyeWidth-10, eyeHeight-10);  //draws the second eye
    fill(0, 0, 0);  //defines the colour of the pupils
    ellipse(headXLoc-10, headYLoc-15, eyeWidth/2+10, eyeHeight/2+10);  //draws the first pupil
    ellipse(headXLoc+25, headYLoc-5, eyeWidth/2-10, eyeHeight/2-10);  //draws the second pupil
    ellipse(headXLoc+10, headYLoc+20, eyeWidth/2, eyeHeight/2);
  }



  void move() {
    if (direction==1) {
      bodyXLoc = bodyXLoc + speed;
    }
    if (direction==2) {
      bodyYLoc=bodyYLoc-speed;
    }
    if (direction==3) {
      bodyXLoc=bodyXLoc-speed;
    }
    if (direction==4) {
      bodyYLoc=bodyYLoc+speed;
    }
    if (bodyXLoc<0||bodyXLoc>690||bodyYLoc<0||Y>690) {
      speed=0;
    }
  }

  void keyPressed() {
    if (keyCode==UP) {
      direction=2;
    }
    if (keyCode==LEFT) {
      direction=3;
    }
    if (keyCode== DOWN) {
      direction=4;
    }
    if (keyCode== RIGHT) {
      direction=1;
    }
  }


  boolean intersect (humans h) {
    float distance = dist(bodyXLoc, bodyYLoc, h.bodyXLoc, h.bodyYLoc);
    if (distance < bodyWidth + h.bodyWidth) {
      return true;
    }
    else {
      return false;
    }
  }
}




//--------------------------------


class humans {

  int bodyXLoc;
  int bodyYLoc;
  int bodyWidth;
  int bodyHeight;

  int headWidth;
  float headHeight;
  int headXLoc;
  int headYLoc;
  float headheight;
  int eyeWidth;
  int eyeHeight;

  int speed;
  boolean finished=false;


  humans () {
    bodyXLoc = int(random(1, width));  //randomly generates the start position of the humans
    bodyYLoc = 1;  //ensures the humans are generated at the top
    bodyWidth = 10;  //changes width of the body
    bodyHeight = 20;  //changes height of the body
    speed = int(random(1, 2));  //changes how fast the humans move
  }


  void move() {
    bodyYLoc += speed;  //makes the humans move downwards
  }
  boolean reachedBottom() {
    if (bodyYLoc > height + bodyHeight) {
      return true;
    }
    else {
      return false;
    }
  }

  void drawBody() {
    fill(255, 0, 0);  //defines the colour for the human's body
    rect(bodyXLoc, bodyYLoc, bodyWidth, bodyHeight);  //draws the body
    stroke(255, 0, 0);
    strokeWeight(2);
    line(bodyXLoc+10, bodyYLoc+5, bodyXLoc+20, bodyYLoc+10);
    line(bodyXLoc, bodyYLoc+5, bodyXLoc-10, bodyYLoc+10);
    line(bodyXLoc+10, bodyYLoc+20, bodyXLoc+15, bodyYLoc+30);
    line(bodyXLoc, bodyYLoc+20, bodyXLoc-5, bodyYLoc+30);
    ellipse(bodyXLoc+5, bodyYLoc-5, bodyWidth, bodyWidth);
  }

  void finished() {
    finished = true;
  }
}

sure!

take it out of the class

under draw()

I'm on it

it should look

void keyPressed() {
  if (key==UP) {
    myalien.direction=2;
  }
  if (key==LEFT) {
    myalien.direction=3;
  }
  if (key==DOWN) {
    myalien.direction=4;
  }
  if (key==RIGHT) {
    myalien.direction=1;
  }
}

agreed.

I'm on it.

do you change speed or dir elsewhere?

it's a very very good nice Code you wrote. Congrats!


let me mention the control of the borders. You can get the screen size with width and height

    if (bodyXLoc<42||bodyXLoc>width||
      bodyYLoc<0||bodyYLoc>height-bodyHeight) {
      speed=0;
    }

you are welcome!

It looks very nice!


To start the humans at once

in setup have
  mytimer = new Timer(2);
  // mytimer.start();


in draw()
After

        totalHumans++;
insert
        mytimer.setTime(3000);

why does the score counter dissapear when there are no humans?

You should make it so it's always visible

I like it a lot.

Alien eating humans.

Rocks!

Very Good!