オペランド型が不正です。はどうすれば良いのでしょうか

前提・実現したいこと

MStringObj の #trim を実装したいのですが、以下のエラーが発生します。空白があることを表したいのですが、うまくいきません。ちなみに、 "" でもやってみたのですが、だめでした。

発生している問題・エラーメッセージ

MStringObj.java:164: エラー: 二項演算子'=='のオペランド型が不正です
        } else if (chars[1] == null){
                            ^
  最初の型: char
  2番目の型: <null>
エラー1個

該当のソースコード(MStringObj)

Java
1public class MStringObj implements MString {
2
3    private char[] chars;
4
5    public MStringObj(char[] chars) {
6        if (chars == null) {
7            throw new IllegalArgumentException();
8        }
9	
10        this.chars = chars;
11    }
12
13    public MStringObj() {
14        this.chars = new char[0];
15    }
16
17    public char[] getChars() {
18        return this.chars;
19    }
20
21    public char charAt(int i) {
22        char[] a = new char[i];
23        return a[i];
24    }
25    
26    public int length() {
27        return this.chars.length;
28    }
29
30    public boolean contains(char c) {
31        for (int i = 0; i < chars.length; i++) {
32            if (chars[i] == c) {
33                return true;
34            }
35        }
36        return false;
37    }
38
39    public int count(char c) {
40        int x = 0;
41        for (int i = 0; i < chars.length; i++) {
42            if (chars[i] == c) {
43                x++;
44            }
45            return x;
46        }
47    }
48
49    public char[] toCharArray() {
50        return this.chars;
51    }
52
53    public MString append(MString s) {
54        char[] a = s.toCharArray();
55        char[] b = new char[this.chars.length + a.length];
56        return new MStringObj(b);
57    }
58
59    public MString replace(MString find, MString replace) {
60        if (chars.length == 0) {
61            return null;
62        } else {
63            char[] a = find.toCharArray();
64            char[] b = replace.toCharArray();
65            char[] c = new char[this.chars.length - a.length + b.length];
66            return new MStringObj(c);
67        }
68    }
69
70    public Integer firstOccurrence(char c) {
71        int x = x;
72        if (this.contains(c) == false) {
73            return null;
74        } else {
75            for (int i = 0; i < chars.length; i++) {
76                if (chars[i] == c) {
77                    x = i;
78                }
79            return x;
80            }
81        }
82    }
83
84    public Integer firstOccurrence(MString s) {
85        int x = x;
86        char[] a = s.toCharArray();
87        if (a.length == 0) {
88            return null;
89        }
90        if (chars.length == a.length) {
91            return null;
92        } else {
93            for (int i = 0; i < chars.length; i++) {
94                x = i;
95                return x;
96            }
97        }
98    }
99
100    public MString substring(int start, int len) {
101        if (chars.length == 0) {
102            return null;
103        } else {
104            char[] a = new char[start];
105            char[] b = new char[len];
106            char[] c = new char[b.length - a.length];
107            return new MStringObj(c);
108        }
109    }
110
111    public char[] toUpperCase() {
112        return this.chars;
113    }
114
115    public char[] toLowerCase() {
116        return this.chars;
117    }
118
119    public MString uppercase() {
120        if (this.chars.length == 0) {
121            return null;
122        } else {
123            return new MStringObj(this.toUpperCase());
124        }
125    }
126
127    public MString lowercase() {
128        if (this.chars.length == 0) {
129            return null;
130        } else {
131            return new MStringObj(this.toLowerCase());
132        }
133    }
134
135    public MString truncate(int n) {
136        if (chars.length <= n) {
137            return null;
138        } else {
139            char[] a = new char[n];
140            return new MStringObj(a);
141        }
142    }
143
144    public boolean palindrome() {
145        if (chars.length == 0) {
146            return false;
147        } else {
148            int a = 0;
149            int b = chars.length - 1;
150            while (a < b) {
151                return false;
152            }
153            a++;
154            b--;
155        }
156        return true;
157    }
158
159    public MString trim() {
160        char[] a = new char[1];
161        char[] b = new char[chars.length - 1];
162        if (chars.length == 0) {
163            return null;
164        } else if (chars[1] == null){
165            char[] c = new char[chars.length - a.length];
166            return new MStringObj(c);
167        } else {
168            char[] d = new char[chars.length - b.length];
169            return new MStringObj(d);
170        }
171    }
172
173    public MString collapse() {
174        char[] a = new char[1];
175        char[] b = new char[chars.length - 1];
176        if (chars.length == 0) {
177            return null;
178        } else {
179            char[] c = new char[chars.length - a.length -b.length];
180            return new MStringObj(c);
181        }
182    }
183
184}