Try to solve these problems yourself. Only come look here if your code doesn't work, and you want to see what you did wrong.

Nested Loops

public static void main() {
int doing, best, chose, score, sum;
boolean Xwon = false, Owon = false, cats = false;
int[] board = new int[10];
for (int nx=1;nx<=9;nx++) board[nx] = 0;
for (int max=0;max<9;max++) { // do one play from each player..

// 1. Display the board..
doing = 0;
for (int ro=1;ro<=3;ro++) { // do three rows..
// prepare to display one row..
System.out.println("");
if (ro>1) System.out.println("---+---+---");
for (int co=1;co<=3;co++) { // do 3 squares in that row..
// display one square...
if (co>1) System.out.print(" | ");
else System.out.print(" ");
doing++;
if (board[doing]>0) System.out.print("X");
else if (board[doing]<0) System.out.print("O");
else System.out.print(doing);
} // end of 3 squares
} // end of 3 rows
System.out.println("");
System.out.println("");

// 2. Is game over?
if (Xwon) break;
if (Owon) break;
cats = true;
for (int nx=1;nx<=9;nx++) if (board[nx]==0) cats = false;
if (cats) break;
System.out.println(""); // blank line for clarity

// 3. Accept X play
System.out.print("Your play: ");
while (true) {
doing = Zystem.ReadInt();
if (doing>9) doing = 0; // so a single test catches all off-board plays
if (doing<1) break; // invalid input, user wants out
if (board[doing] == 0) { // valid play..
board[doing]++; // mark X as having played this square
break;}
System.out.print("Taken! "); // ..then go back for another input
} // end of input loop
if (doing<1) break; // invalid input
if (board[1]>0) if (board[2]>0) if (board[3]>0) Xwon = true; // top row
if (board[4]>0) if (board[5]>0) if (board[6]>0) Xwon = true; // middle row
if (board[7]>0) if (board[8]>0) if (board[9]>0) Xwon = true; // bottom row
if (board[1]>0) if (board[4]>0) if (board[7]>0) Xwon = true; // left column
if (board[2]>0) if (board[5]>0) if (board[8]>0) Xwon = true; // middle column
if (board[3]>0) if (board[6]>0) if (board[9]>0) Xwon = true; // right column
if (board[1]>0) if (board[5]>0) if (board[9]>0) Xwon = true; // down diagonal
if (board[3]>0) if (board[5]>0) if (board[7]>0) Xwon = true; // up diagonal
if (Xwon) continue; // display final board then exit
if (max == 4) continue; // ditto if cats

// 4. Calculate O play
best = -1;
chose = 0;
for (doing=1;doing<=9;doing++) { // try all available squares
if (board[doing] !=0) continue; // already played, go to next square
score = 0;
if (doing<4) { // in the top row..
sum = board[1]+board[2]+board[3];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of top row test
else if (doing>6) { // in the bottom row..
sum = board[7]+board[8]+board[9];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of bottom row test
else if (doing>3) if (doing<7) { // in the middle row (last test)..
sum = board[4]+board[5]+board[6];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of middle row test
if (doing%3 == 0) { // in the right column..
sum = board[3]+board[6]+board[9];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of right column test
else if (doing%3 == 1) { // in the left column..
sum = board[1]+board[4]+board[7];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of left column test
else if (doing%3 == 2) { // in the middle column..
sum = board[2]+board[5]+board[8];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of middle column test
if (doing%4 == 1) { // in the down diagonal..
sum = board[1]+board[5]+board[9];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of down diagonal test
else if ((doing==3)||(doing==5)||(doing==7)) { // in the up diagonal..
sum = board[3]+board[5]+board[7];
if (sum == -4) score = 99;
else if (sum == 2) score = score+22;
else if (sum == 0) score++;
else if (sum == -2) score = score+4;
else if (sum == 1) score = score+2;
} // end of up diagonal test
if (score>best) { // best square so far..
chose = doing;
best = score;} // otherwise ignore this square, we already have better
} // end of heuristic loop
if (chose==0) { // huh? this shouldn't happen..
System.out.println("Something went wrong");
break;}
board[chose] = -2; // mark O as having played this square
if (best>= 99) Owon = true; // did O win?
// if (board[1]<0) if (board[2]<0) if (board[3]<0) Owon = true; // top row
// if (board[4]<0) if (board[5]<0) if (board[6]<0) Owon = true; // middle row
// if (board[7]<0) if (board[8]<0) if (board[9]<0) Owon = true; // bottom row
// if (board[1]<0) if (board[4]<0) if (board[7]<0) Owon = true; // left column
// if (board[2]<0) if (board[5]<0) if (board[8]<0) Owon = true; // middle column
// if (board[3]<0) if (board[6]<0) if (board[9]<0) Owon = true; // right column
// if (board[1]<0) if (board[5]<0) if (board[9]<0) Owon = true; // down diagonal
// if (board[3]<0) if (board[5]<0) if (board[7]<0) Owon = true; // up diagonal
} // end of double-play for
// congratulate winner
if (Xwon) System.out.println("You won, oh well.");
else if (Owon) System.out.println("I won, I won, I won!");
else System.out.println("Cat's game.");
} // end of main

Methodical Code Re-Use

public static int Add2score(int eX, int Oh, int both, int aBit, int mask) {
if ((mask&aBit) ==0) return 0; // the current square is not in this row
if (((both|aBit)&mask) == mask) return 0; // this row has both X and O
if ((both&mask)==0) return 1; // the row is empty
if ((Oh&mask)==0) return 2; // it has one X only
if ((eX&mask)==0) return 4; // it has one O only
return 0; // probably can't get here, but the compiler requires a return
} // end of Add2score

public static boolean Winner(int playlist) { // <-- header
if ((playlist&0xE)==0xE) return true; // top row wins
if ((playlist&0x70)==0x70) return true; // middle row
if ((playlist&0x380)==0x380) return true; // bottom row
if ((playlist&0x92)==0x92) return true; // left column
if ((playlist&0x124)==0x124) return true; // middle col
if ((playlist&0x248)==0x248) return true; // right column
if ((playlist&0x222)==0x222) return true; // down diagonal
if ((playlist&0xA8)==0xA8) return true; // up diagonal
return false; // there is no winning three
} // end of Winner

public static void main() {
int eX = 0, Oh = 0, both = 0, doing, aBit, best, chose, both, score, mask;
boolean Xwon = false, Owon = false, logit = false;
String aLine = "";
for (int max=0;max<9;max++) { // do one play from each player..
both = eX|Oh;

// 1. Display the board..
aBit = 1;
doing = 0;
for (int ro=1;ro<=3;ro++) { // do three rows..
// prepare to display one row..
System.out.println("");
if (ro>1) System.out.println("---+---+---");
for (int co=1;co<=3;co++) { // do 3 squares in that row..
// display one square...
if (co>1) System.out.print(" | ");
else System.out.print(" ");
aBit = aBit<<1;
doing++;
if ((eX&aBit) != 0) System.out.print("X");
else if ((Oh&aBit) != 0) System.out.print("O");
else System.out.print(doing);
} // end of 3 squares
} // end of 3 rows
System.out.println("");
System.out.println("");

// 2. Is game over?
if (Xwon) break;
if (Owon) break;
if (eX+Oh == 0x3FE) break;
System.out.println(""); // blank line for clarity

// 3. Accept X play
System.out.print("Your play: ");
while (true) {
doing = Zystem.ReadInt();
if (doing>9) doing = 0; // so a single test catches all off-board plays
if (doing<1) break; // invalid input, user wants out
aBit = 1<<doing;
if (((eX|Oh)&aBit) == 0) { // valid play..
eX = eX|aBit; // mark X as having played this square
break;}
System.out.print("Taken! "); // ..then go back for another input
} // end of input loop
if (doing<1) break; // invalid input
aBit = 1<<doing;
eX = eX|aBit;
both = eX|Oh;
if (Winner(eX)) Xwon = true;
if (Xwon) continue; // display final board then exit
if (both == 0x3FE) continue; // ditto if cats

// 4. Calculate O play
aBit = 1;
best = -1;
chose = 0;
for (doing=1;doing<=9;doing++) { // try all available squares
aBit = aBit<<1;
if ((both&aBit) !=0) continue; // already played, go to next square
score = 0;
if (Winner(Oh|aBit)) score = 99; // yay! (a winner)
if (Winner(eX|aBit)) score = score+22; // prevent a lose, block this one
// for each row/column/diagonal doing is in, if doing does not fill it...
score = Add2score(eX,Oh,both,aBit,0xE)+score; // top row
score = Add2score(eX,Oh,both,aBit,0x70)+score; // 2nd row
score = Add2score(eX,Oh,both,aBit,0x380)+score; // bottom row
score = Add2score(eX,Oh,both,aBit,0x92)+score; // left column
score = Add2score(eX,Oh,both,aBit,0x124)+score; // middle col
score = Add2score(eX,Oh,both,aBit,0x248)+score; // right column
score = Add2score(eX,Oh,both,aBit,0xA8)+score; // up diagonal
score = Add2score(eX,Oh,both,aBit,0x222)+score; // down diagonal
if (score>best) { // best square so far..
chose = doing;
best = score;} // otherwise ignore this square, we already have better
} // end of heuristic loop
if (chose==0) { // huh? this shouldn't happen..
System.out.println("Something went wrong");
break;}
aBit = 1<<chose;
Oh = Oh|aBit; // mark O as having played this square
// did O win?
if (Winner(Oh)) Owon = true;
} // end of double-play while
// congratulate winner
if (Xwon) System.out.println("You won, oh well.");
else if (Owon) System.out.println("I won, I won, I won!");
else if (both == 0x3FE) System.out.println("Cat's game.");
} // end of main

Exchange

A = B-A;
B = B-A;
A = B+A;

Revised: 2020 July 22